[#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>

object/grpc/service_frostfs.pb.go

@@ -924,7 +924,13 @@ func (x *GetResponse_Body) UnmarshalEasyJSON(in *jlexer.Lexer) {
 			x.ObjectPart = xx
 			{
 				var f []byte
-				f = in.Bytes()
+				{
+					tmp := in.Bytes()
+					if len(tmp) == 0 {
+						tmp = nil
+					}
+					f = tmp
+				}
 				xx.Chunk = f
 			}
 		case "splitInfo":
@@ -1699,7 +1705,13 @@ func (x *PutRequest_Body) UnmarshalEasyJSON(in *jlexer.Lexer) {
 			x.ObjectPart = xx
 			{
 				var f []byte
-				f = in.Bytes()
+				{
+					tmp := in.Bytes()
+					if len(tmp) == 0 {
+						tmp = nil
+					}
+					f = tmp
+				}
 				xx.Chunk = f
 			}
 		}
@@ -6254,7 +6266,13 @@ func (x *GetRangeResponse_Body) UnmarshalEasyJSON(in *jlexer.Lexer) {
 			x.RangePart = xx
 			{
 				var f []byte
-				f = in.Bytes()
+				{
+					tmp := in.Bytes()
+					if len(tmp) == 0 {
+						tmp = nil
+					}
+					f = tmp
+				}
 				xx.Chunk = f
 			}
 		case "splitInfo":
@@ -6802,7 +6820,13 @@ func (x *GetRangeHashRequest_Body) UnmarshalEasyJSON(in *jlexer.Lexer) {
 		case "salt":
 			{
 				var f []byte
-				f = in.Bytes()
+				{
+					tmp := in.Bytes()
+					if len(tmp) == 0 {
+						tmp = nil
+					}
+					f = tmp
+				}
 				x.Salt = f
 			}
 		case "type":
@@ -7267,7 +7291,13 @@ func (x *GetRangeHashResponse_Body) UnmarshalEasyJSON(in *jlexer.Lexer) {
 				var list [][]byte
 				in.Delim('[')
 				for !in.IsDelim(']') {
-					f = in.Bytes()
+					{
+						tmp := in.Bytes()
+						if len(tmp) == 0 {
+							tmp = nil
+						}
+						f = tmp
+					}
 					list = append(list, f)
 					in.WantComma()
 				}
@@ -8455,7 +8485,13 @@ func (x *PatchRequest_Body_Patch) UnmarshalEasyJSON(in *jlexer.Lexer) {
 		case "chunk":
 			{
 				var f []byte
-				f = in.Bytes()
+				{
+					tmp := in.Bytes()
+					if len(tmp) == 0 {
+						tmp = nil
+					}
+					f = tmp
+				}
 				x.Chunk = f
 			}
 		}