Protobuf parse should support parsing JSON object containing JSON Null.

(This bug was exposed by MESOS-4184, when serializing docker v1 image manifest as protobuf).

Currently protobuf::parse returns failures when parsing any JSON containing JSON::Null. If we have any protobuf field set as `JSON::Null`, any other non-repeated field cannot capture their value. For example, assuming we have a protobuf message:

message Nested {
  optional string str = 1;
  repeated string json_null = 2;
}

If there exists any field containing JSON::Null, like below:

      {
        \"str\": \"message\",
        \"json_null\": null
      }

When we do protobuf::parse, it would return the following failure:

Failure parse: Not expecting a JSON null