Send empty inline response to IME if augmented autofill returns no suggestion

* The augmented autofill may dynamically request an autofill request
  which will "invalidate" the old suggestions. In case the new request
  doesn't return any suggestions, we need to make sure the old
  suggestions are removed from the IME.
* See the scenario in https://b.corp.google.com/issues/158038231#comment14

Test: manual
Test: atest android.autofillservice.cts.inline
Bug: 157515522
Bug: 158038231

Change-Id: If85592395ad918197566a5ca556fba8ccc971071