rpc: emit Null in case of Any parameter with zero-len value
Otherwise it leads to the following error in the TestActor_CallWithNilParam:
```
=== RUN TestActor_CallWithNilParam
logger.go:130: 2023-04-03T15:58:27.672+0300 INFO initial gas supply is not set or wrong, setting default value {"InitialGASSupply": "52000000"}
logger.go:130: 2023-04-03T15:58:27.672+0300 INFO P2PNotaryRequestPayloadPool size is not set or wrong, setting default value {"P2PNotaryRequestPayloadPoolSize": 1000}
logger.go:130: 2023-04-03T15:58:27.672+0300 INFO MaxBlockSize is not set or wrong, setting default value {"MaxBlockSize": 262144}
logger.go:130: 2023-04-03T15:58:27.672+0300 INFO MaxBlockSystemFee is not set or wrong, setting default value {"MaxBlockSystemFee": 900000000000}
logger.go:130: 2023-04-03T15:58:27.672+0300 INFO MaxTransactionsPerBlock is not set or wrong, using default value {"MaxTransactionsPerBlock": 512}
logger.go:130: 2023-04-03T15:58:27.672+0300 INFO MaxValidUntilBlockIncrement is not set or wrong, using default value {"MaxValidUntilBlockIncrement": 5760}
logger.go:130: 2023-04-03T15:58:27.675+0300 INFO no storage version found! creating genesis block
logger.go:130: 2023-04-03T15:58:27.675+0300 INFO ExtensiblePoolSize is not set or wrong, using default value {"ExtensiblePoolSize": 20}
logger.go:130: 2023-04-03T15:58:27.675+0300 INFO SessionPoolSize is not set or wrong, setting default value {"SessionPoolSize": 20}
logger.go:130: 2023-04-03T15:58:27.675+0300 INFO MaxWebSocketClients is not set or wrong, setting default value {"MaxWebSocketClients": 64}
logger.go:130: 2023-04-03T15:58:27.675+0300 INFO starting rpc-server {"endpoint": "localhost:0"}
logger.go:130: 2023-04-03T15:58:27.677+0300 DEBUG done processing headers {"headerIndex": 1, "blockHeight": 0, "took": "436.313µs"}
logger.go:130: 2023-04-03T15:58:27.679+0300 DEBUG done processing headers {"headerIndex": 2, "blockHeight": 1, "took": "272.891µs"}
logger.go:130: 2023-04-03T15:58:27.680+0300 DEBUG done processing headers {"headerIndex": 3, "blockHeight": 2, "took": "276.949µs"}
logger.go:130: 2023-04-03T15:58:27.681+0300 DEBUG done processing headers {"headerIndex": 4, "blockHeight": 3, "took": "286.028µs"}
logger.go:130: 2023-04-03T15:58:27.681+0300 DEBUG done processing headers {"headerIndex": 5, "blockHeight": 4, "took": "268.673µs"}
logger.go:130: 2023-04-03T15:58:27.681+0300 INFO bad notification {"contract": "565cff9508ebc75aadd7fe59f38dac610ab6093c", "event": "Transfer", "error": "parameter 0 type mismatch: Hash160 vs ByteString"}
logger.go:130: 2023-04-03T15:58:27.682+0300 DEBUG done processing headers {"headerIndex": 6, "blockHeight": 5, "took": "380.988µs"}
logger.go:130: 2023-04-03T15:58:27.683+0300 DEBUG done processing headers {"headerIndex": 7, "blockHeight": 6, "took": "273.543µs"}
logger.go:130: 2023-04-03T15:58:27.683+0300 DEBUG done processing headers {"headerIndex": 8, "blockHeight": 7, "took": "275.163µs"}
logger.go:130: 2023-04-03T15:58:27.684+0300 DEBUG done processing headers {"headerIndex": 9, "blockHeight": 8, "took": "259.578µs"}
logger.go:130: 2023-04-03T15:58:27.685+0300 DEBUG done processing headers {"headerIndex": 10, "blockHeight": 9, "took": "266.882µs"}
logger.go:130: 2023-04-03T15:58:27.686+0300 DEBUG done processing headers {"headerIndex": 11, "blockHeight": 10, "took": "295.3µs"}
logger.go:130: 2023-04-03T15:58:27.687+0300 DEBUG done processing headers {"headerIndex": 12, "blockHeight": 11, "took": "295.568µs"}
logger.go:130: 2023-04-03T15:58:27.688+0300 DEBUG done processing headers {"headerIndex": 13, "blockHeight": 12, "took": "258.197µs"}
logger.go:130: 2023-04-03T15:58:27.689+0300 DEBUG done processing headers {"headerIndex": 14, "blockHeight": 13, "took": "261.602µs"}
logger.go:130: 2023-04-03T15:58:27.689+0300 DEBUG done processing headers {"headerIndex": 15, "blockHeight": 14, "took": "268.922µs"}
logger.go:130: 2023-04-03T15:58:27.690+0300 DEBUG done processing headers {"headerIndex": 16, "blockHeight": 15, "took": "276.176µs"}
logger.go:130: 2023-04-03T15:58:27.691+0300 DEBUG done processing headers {"headerIndex": 17, "blockHeight": 16, "took": "256.068µs"}
logger.go:130: 2023-04-03T15:58:27.692+0300 DEBUG done processing headers {"headerIndex": 18, "blockHeight": 17, "took": "262.303µs"}
logger.go:130: 2023-04-03T15:58:27.692+0300 DEBUG done processing headers {"headerIndex": 19, "blockHeight": 18, "took": "265.087µs"}
logger.go:130: 2023-04-03T15:58:27.693+0300 DEBUG done processing headers {"headerIndex": 20, "blockHeight": 19, "took": "260.758µs"}
logger.go:130: 2023-04-03T15:58:27.694+0300 DEBUG done processing headers {"headerIndex": 21, "blockHeight": 20, "took": "263.482µs"}
logger.go:130: 2023-04-03T15:58:27.694+0300 DEBUG done processing headers {"headerIndex": 22, "blockHeight": 21, "took": "327.812µs"}
logger.go:130: 2023-04-03T15:58:27.696+0300 DEBUG done processing headers {"headerIndex": 23, "blockHeight": 22, "took": "284.104µs"}
logger.go:130: 2023-04-03T15:58:27.697+0300 WARN contract invocation failed {"tx": "82279bfe9bada282ca0f8cb8e0bb124b921af36f00c69a518320322c6f4fef60", "block": 23, "error": "at instruction 0 (ABORT): ABORT"}
logger.go:130: 2023-04-03T15:58:27.697+0300 DEBUG processing rpc request {"method": "getversion", "params": "[]"}
logger.go:130: 2023-04-03T15:58:27.698+0300 DEBUG processing rpc request {"method": "invokefunction", "params": "[565cff9508ebc75aadd7fe59f38dac610ab6093c putValue ]"}
client_test.go:2562:
Error Trace: /home/anna/Documents/GitProjects/nspcc-dev/neo-go/pkg/services/rpcsrv/client_test.go:2562
Error: Should be true
Test: TestActor_CallWithNilParam
Messages: at instruction 6 (PACK): OPACK: invalid length
logger.go:130: 2023-04-03T15:58:27.699+0300 INFO shutting down RPC server {"endpoint": "127.0.0.1:46005"}
logger.go:130: 2023-04-03T15:58:27.700+0300 INFO persisted to disk {"blocks": 23, "keys": 1236, "headerHeight": 23, "blockHeight": 23, "took": "908.825µs"}
--- FAIL: TestActor_CallWithNilParam (0.03s)
FAIL
```
See also the ref. df534f6b0c/src/Neo/SmartContract/ContractParameter.cs (L141)
and the way how parameters are handled by ref. RPC server:
4b3a76e1b7/src/RpcServer/RpcServer.SmartContract.cs (L202)
and FromJSON implementation:
df534f6b0c/src/Neo/SmartContract/ContractParameter.cs (L70)
01ac2d9f31