[#120] protogen: Omit empty fields from JSON output

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

util/protogen/internalgengo/json.go

@@ -147,8 +147,11 @@ func emitJSONMarshal(g *protogen.GeneratedFile, msg *protogen.Message) {
 	g.P("func (x *", msg.GoIdent.GoName, ") MarshalEasyJSON(out *", jwriterPackage.Ident("Writer"), ") {")
 	g.P(`if x == nil { out.RawString("null"); return }`)
 
+	if len(msg.Fields) != 0 {
+		g.P("first := true")
+	}
 	g.P("out.RawByte('{')")
-	for i, f := range msg.Fields {
+	for _, f := range msg.Fields {
 		if f.Oneof != nil {
 			if f.Oneof.Fields[0] != f {
 				continue
@@ -157,29 +160,35 @@ func emitJSONMarshal(g *protogen.GeneratedFile, msg *protogen.Message) {
 			g.P("switch xx := x.", f.Oneof.GoName, ".(type) {")
 			for _, ff := range f.Oneof.Fields {
 				g.P("case *", ff.GoIdent, ":")
-				emitJSONFieldWrite(g, ff, "xx", i == 0)
+				emitJSONFieldWrite(g, ff, "xx")
 			}
 			g.P("}")
 			continue
 		}
-		emitJSONFieldWrite(g, f, "x", i == 0)
+		emitJSONFieldWrite(g, f, "x")
 	}
 	g.P("out.RawByte('}')")
 	g.P("}")
 }
 
-func emitJSONFieldWrite(g *protogen.GeneratedFile, f *protogen.Field, name string, first bool) {
+func emitJSONFieldWrite(g *protogen.GeneratedFile, f *protogen.Field, name string) {
 	g.P("{")
 	defer g.P("}")
 
-	g.P("const prefix string = ", `",\"`, fieldJSONName(f), `\":"`)
-	if first {
-		g.P("out.RawString(prefix[1:])")
-	} else {
-		g.P("out.RawString(prefix)")
-	}
-
 	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("}")
+
+	g.P("if !first { out.RawByte(','); } else { first = false; }")
+	g.P("const prefix string = ", `"\"`, fieldJSONName(f), `\":"`)
+	g.P("out.RawString(prefix)")
 	if f.Desc.IsList() {
 		selector += "[i]"
 		g.P("out.RawByte('[')")