Don't import PO files with no translated string from Transifex
_Originally created by @intrigeri on [#17106 (Redmine)](https://public-redmine-archive.tails.boum.org/code/issues/17106)_
On tails/tails#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 tails/tails#16095 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: tails/tails#16774
### Related issues
- **Blocks** tails/tails#16095
issue