Fixed NPE when FillService replies with a null FillResponse.

This problem was not detected by CTS tests because the exception is
"harmless" (it's just ignored), but it can be verified at logcat:

W Binder  : Caught a RuntimeException from the binder stub implementation.
W Binder  : java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.IntentSender android.service.autofill.FillResponse.getAuthentication()' on a null object reference
W Binder  : 	at com.android.server.autofill.ViewState.maybeCallOnFillReady(ViewState.java:159)

Bug: 35707731
Test: CtsAutoFillServiceTestCases pass
Test: manual verification
Change-Id: I58330a7e91b2c186bf97897b7d39640010ca630b