Don't import PO files with no translated string from Transifex

On #16774 we decided we want to import from Transifex:

• only reviewed translations → this was done for the tails-misc resource on the tails-misc_release branch
• only PO files with at least one translated+reviewed string → emmapeel tells me she can’t do this on Tor’s side, so we need to filter out these PO files in import-translations

This matters because the code on #16095 (closed) will make decisions based on which PO files are present in tails.git:po/, and it assumes that the aforementioned criteria is implemented there.

Feature Branch: bugfix/17106-dont-import-empty-po-files

Parent Task: #16774

Related issues

• Blocks #16209
• Blocks #16095 (closed)

Original created by @intrigeri on 17106 (Redmine)