[#107] protogen: Fix oneof JSON marshaling

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

util/protogen/internalgengo/json.go

@@ -159,8 +159,6 @@ func emitJSONMarshal(g *protogen.GeneratedFile, msg *protogen.Message) {
 				g.P("case *", ff.GoIdent, ":")
 				emitJSONFieldWrite(g, ff, "xx", i == 0)
 			}
-			g.P("default:")
-			g.P(`out.RawString("null")`)
 			g.P("}")
 			continue
 		}