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.

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

util/proto/marshal_test.go

@@ -50,11 +50,7 @@ func TestStableMarshalSingle(t *testing.T) {
 
 			var actualFrostfs generated.Primitives
 			require.NoError(t, actualFrostfs.UnmarshalJSON(r))
-			if len(actualFrostfs.FieldA) == 0 {
-				actualFrostfs.FieldA = nil
-			}
 			require.Equal(t, input, &actualFrostfs)
-
 			primitivesEqual(t, input, &actual)
 		})
 	})
@@ -110,12 +106,8 @@ func TestStableMarshalSingle(t *testing.T) {
 
 				var actualFrostfs generated.Primitives
 				require.NoError(t, actualFrostfs.UnmarshalJSON(r))
-				if len(actualFrostfs.FieldA) == 0 {
-					actualFrostfs.FieldA = nil
-				}
 				require.Equal(t, tc.input, &actualFrostfs)
-
-				primitivesEqual(t, tc.input, &actual)
+				primitivesEqual(t, &actualFrostfs, &actual)
 			})
 		})
 	}
@@ -124,9 +116,7 @@ func TestStableMarshalSingle(t *testing.T) {
 func primitivesEqual(t *testing.T, a *generated.Primitives, b *test.Primitives) {
 	// Compare each field directly, because proto-generated code has private fields.
 	require.Equal(t, len(a.FieldA), len(b.FieldA))
-	if len(a.FieldA) != 0 {
 	require.Equal(t, a.FieldA, b.FieldA)
-	}
 	require.Equal(t, a.FieldB, b.FieldB)
 	require.Equal(t, a.FieldC, b.FieldC)
 	require.Equal(t, a.FieldD, b.FieldD)

util/protogen/internalgengo/json.go

@@ -129,7 +129,17 @@ func emitJSONFieldRead(g *protogen.GeneratedFile, f *protogen.Field, name string
 	case protoreflect.StringKind:
 		template = "%s = in.String()"
 	case protoreflect.BytesKind:
-		template = "%s = in.Bytes()"
+		// Since some time ago proto3 support optional keyword, thus the presence is not tracked by default:
+		// https://github.com/protocolbuffers/protobuf-go/blob/fb995f184a1719ec42b247a3771d1036d92adf67/internal/impl/message_reflect_field.go#L327
+		// We do not explicitly support `optional` keyword, protoc will fail on such fileds.
+		// Thus, treat empty string as `[]byte(nil)`.
+		template = `{
+			tmp := in.Bytes()
+			if len(tmp) == 0 {
+				tmp = nil
+			}
+			%s = tmp
+		}`
 	case protoreflect.MessageKind:
 		if f.Desc.IsList() {
 			g.P("f = ", fieldType(g, f)[2:], "{}")