Changed onGetScores() to return null instead of throwing exception.

Callers (Session and AFMShellCommand) already handle null (in fact, the
signature declares it as @Nullable), so there's no point on throwing a
runtime exception...

Bug: 70939974
Test: 'adb shell cmd autofill get fc_score half kale' when service APK was not
      implementing it.

Change-Id: I37da6cd13c40545626332c272759e797b5a25924