Fix AutofillValueTest#autofillEmptyTextValue.

The new added API for rich content(ag/9884022) denies to autofill
empty text. However, Autofill allows us to fill an empty text. We
should not cause a compatibility issue.

Bug: 159035197
Test: atest android.autofillservice.cts.AutofillValueTest#\
autofillEmptyTextValue
Test: atest FrameworksCoreTests:AutofillValueTest

Change-Id: Ieb5b82bfe4bf77abd151c61ea4cf9125b8fe61f5