Return an empty list instead of null.

TextServicesManager#getEnabledSpellCheckerInfos() was introduced in
  commit 4c742373
as #getEnabledSpellCheckersList().
Then, the following commit added SuppressLint("NullableCollections"):
  commit d7f33cd6

As this API has been introduced recently in Android S timeframe, I think
it's better to fix the API behavior before it's finalized.

Bug: 180625329
Test: m checkapi
Change-Id: I33d3c86a0c19c0082299e383e50d3860b072e79d