[#123] protogen: Treat bytes field as non-nullable

In protobuf 3.12 they have added an support for `optional` keyword,
which has made it into the main branch in 3.15.
https://github.com/protocolbuffers/protobuf/blob/main/docs/implementing_proto3_presence.md
https://github.com/protocolbuffers/protobuf/blob/v3.12.0/docs/field_presence.md#presence-in-proto3-apis

This means that without an explicit `optional` keyword field presence
for scalars is not tracked, thus empty string in JSON should be
unmarshaled to a nil byte slice. Relevant decoding code and tests from
protojson:
fb995f184a/internal/impl/message_reflect_field.go (L327)
fb995f184a/encoding/protojson/decode_test.go (L134)
fb995f184a/encoding/protojson/decode_test.go (L156)

We do not support `optional` keyword and the generator will fail if it sees on.
So only implement the default behaviour.

Refs #122

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

container/grpc/types_frostfs.pb.go

@@ -500,7 +500,13 @@ func (x *Container) UnmarshalEasyJSON(in *jlexer.Lexer) {
 		case "nonce":
 			{
 				var f []byte
-				f = in.Bytes()
+				{
+					tmp := in.Bytes()
+					if len(tmp) == 0 {
+						tmp = nil
+					}
+					f = tmp
+				}
 				x.Nonce = f
 			}
 		case "basicACL":