core: fix bug in nep5 notifications handling

Native contracts deployment creates `Transfer` notifications and adds
them into interop context. However, these notifications were not stored
for two reasons:
1. typo in `Transfer` (so these notifications were not recognised during
processing of the invocation tx in (*Blockchain).storeBlock(...) method)
2. these notifications have `from` adress setted to null, so conversion
to []byte fails. Same thing could happen with `to`.

Related C# issue: https://github.com/neo-project/neo/issues/1646

For now, made both `transfer` and `Transfer` valid.

pkg/core/blockchain.go

@@ -651,16 +651,26 @@ func (bc *Blockchain) storeBlock(block *block.Block) error {
 						continue
 					}
 					op, ok := arr[0].Value().([]byte)
-					if !ok || string(op) != "transfer" {
+					if !ok || (string(op) != "transfer" && string(op) != "Transfer") {
 						continue
 					}
-					from, ok := arr[1].Value().([]byte)
+					var from []byte
-					if !ok {
+					fromValue := arr[1].Value()
-						continue
+					// we don't have `from` set when we are minting tokens
+					if fromValue != nil {
+						from, ok = fromValue.([]byte)
+						if !ok {
+							continue
+						}
 					}
-					to, ok := arr[2].Value().([]byte)
+					var to []byte
-					if !ok {
+					toValue := arr[2].Value()
-						continue
+					// we don't have `to` set when we are burning tokens
+					if toValue != nil {
+						to, ok = toValue.([]byte)
+						if !ok {
+							continue
+						}
 					}
 					amount, ok := arr[3].Value().(*big.Int)
 					if !ok {