matf/stop-overwriting-state-on-empty-state (#53)

- if the state message is empty (that is, no state message was
  processed) then do not attempt to write the state as this can
  overwrite state in the event a tap produces no records (or no state
  message)

target_bigquery/target.py

@@ -526,5 +526,6 @@ def drain_all(self, is_endofpipe: bool = False) -> None:  # type: ignore
         else:
             for sink in self._sinks_active.values():
                 sink.pre_state_hook()
-        self._write_state_message(state)
+        if state:
+            self._write_state_message(state)
         self._reset_max_record_age()