Handle mutable default arguments cleanly

When generating code, ensure that default list/dict arguments are
initialised in local scope if unspecified or `None`.

tests/inputs/service/service.proto

@@ -4,6 +4,7 @@ package service;
 
 message DoThingRequest {
   string name = 1;
+  repeated string comments = 2;
 }
 
 message DoThingResponse {