Is it possible force save zero integer values with Protocol Buffers in Java?

I'm using Protocol Buffers from Google in my Java projects.

When I define message like that:

message Event {
  int32 action = 1;
}

and save value zero as an action, then value zero won't be saved. It's able to save all integer values except for zero.

I even found it in my decompiled code generated by Protocol Buffers:

...
public void writeTo(CodedOutputStream output) throws IOException {
  if (this.action_ != 0) {
    output.writeInt32(1, this.action_);
  }
  ...
}

Do you know, how can I force saving zeros? Or it's not possible and simply forbidden by Protocol Buffers design?