protogen: Always marshal empty fields

This is how it was done previously:
a0a9b765f3/rpc/message/encoding.go (L31)

The tricky part is `[]byte` which is marshaled as `null` by easyjson
helper, but as `""` by protojson.

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

util/proto/marshal_test.go

@@ -26,12 +26,38 @@ func nonZero[T protoInt]() T {
 
 func TestStableMarshalSingle(t *testing.T) {
 	t.Run("empty", func(t *testing.T) {
+		t.Run("proto", func(t *testing.T) {
 			input := &generated.Primitives{}
 			require.Zero(t, input.StableSize())
 
 			r := input.MarshalProtobuf(nil)
 			require.Empty(t, r)
 		})
+		t.Run("json", func(t *testing.T) {
+			input := &generated.Primitives{}
+			r, err := input.MarshalJSON()
+			require.NoError(t, err)
+			require.NotEmpty(t, r)
+
+			var actual test.Primitives
+			require.NoError(t, protojson.Unmarshal(r, &actual))
+
+			t.Run("protojson compatibility", func(t *testing.T) {
+				data, err := protojson.MarshalOptions{EmitUnpopulated: true}.Marshal(&actual)
+				require.NoError(t, err)
+				require.JSONEq(t, string(data), string(r))
+			})
+
+			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)
+		})
+	})
 
 	marshalCases := []struct {
 		name  string
@@ -77,13 +103,16 @@ func TestStableMarshalSingle(t *testing.T) {
 				require.NoError(t, protojson.Unmarshal(r, &actual))
 
 				t.Run("protojson compatibility", func(t *testing.T) {
-					data, err := protojson.Marshal(&actual)
+					data, err := protojson.MarshalOptions{EmitUnpopulated: true}.Marshal(&actual)
 					require.NoError(t, err)
 					require.JSONEq(t, string(data), string(r))
 				})
 
 				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)
@@ -94,7 +123,10 @@ 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

@@ -192,14 +192,17 @@ func emitJSONFieldWrite(g *protogen.GeneratedFile, f *protogen.Field, name strin
 
 	selector := name + "." + f.GoName
 
-	isNotDefault := notNil
-	if f.Desc.IsList() {
-		isNotDefault = notEmpty
-	} else if f.Desc.Kind() != protoreflect.MessageKind {
-		_, isNotDefault = easyprotoKindInfo(f.Desc.Kind())
-	}
-	g.P("if ", isNotDefault(selector), "{")
-	defer g.P("}")
+	// This code is responsible for ignoring default values.
+	// We will restore it after having parametrized JSON marshaling.
+	//
+	// isNotDefault := notNil
+	// if f.Desc.IsList() {
+	// 	isNotDefault = notEmpty
+	// } else if f.Desc.Kind() != protoreflect.MessageKind {
+	// 	_, isNotDefault = easyprotoKindInfo(f.Desc.Kind())
+	// }
+	// g.P("if ", isNotDefault(selector), "{")
+	// defer g.P("}")
 
 	g.P("if !first { out.RawByte(','); } else { first = false; }")
 	g.P("const prefix string = ", `"\"`, fieldJSONName(f), `\":"`)
@@ -247,7 +250,10 @@ func emitJSONFieldWrite(g *protogen.GeneratedFile, f *protogen.Field, name strin
 	case protoreflect.StringKind:
 		template = "out.String(%s)"
 	case protoreflect.BytesKind:
-		template = "out.Base64Bytes(%s)"
+		g.P("if ", selector, "!= nil {")
+		g.P("out.Base64Bytes(", selector, ")")
+		g.P("} else { out.String(\"\") }")
+		return
 	case protoreflect.MessageKind:
 		template = "%s.MarshalEasyJSON(out)"
 	case protoreflect.GroupKind: