deleteEqualMessages: Exclude messages that are empty by default

The main purpose of this script is to purge local overrides to
avoid them from blocking an i18n update in the future.

However empty messages should remain empty and often have a
richer edit history (e.g. Sitenotice) and may not stay empty
for long.

Change-Id: I5b416cda25a3641862df9919c46ae59ad5d5d6e0
Merge "deleteEqualMessages: Exclude messages that are empty by default"