delete "return null",delivery deserializer to handle

TreeTypeAdapter.java
/**
 @Override public T read(JsonReader in) throws IOException {
    if (deserializer == null) {
      return delegate().read(in);
    }
    JsonElement value = Streams.parse(in);
    if (value.isJsonNull()) {
      return null;
    }
    return deserializer.deserialize(value, typeToken.getType(), context);
  }
**/