matches_locale no longer accept empty locales in the png file

The legacy png files have an empty line in the end. And the recovery
used to match any missing locale, e.g. "he" with that line and gets an
empty image.

Since the empty image is barely useful, we should just error out and
fall back to the default locale.

This reversed the unit test check added in d17a6885

Bug: 128934634
Test: run locale test with "he" and legacy images, recovery reports
error and doesn't crash even without default locale fall back

Change-Id: Ibdb7dd0b42348de5e392c834cce67ff02be85c24