Description

Fixes the following issues:

• --untranslated-messages-file flag doesn't generate any file when all messages have already been translated. This PR now generates an empty JSON file when this is the case.
• --gen-inputs-and-outputs-list file doesn't contain the untranslated-messages-file. It now will.
• Only print message with missing translations status when unimplemented messages is empty
• The deprecated path has been pointing to the wrong command (flutter generate-localizations instead of flutter gen-l10n). Since this has been broken for some time without any complaints, maybe it's a good time to remove it altogether.

Related Issues

Fixes #67138

Tests

I added the following tests:

• A test to ensure that inputs and outputs list contains the untranslated messages file when specified
• Tests to ensure that the untranslated messages file status correctly generated