checkers/check_via_appstream.py: Do not remove values, that have been filled

Currently, it can happen (I am only aware of it happening with the app author taxonomy (https://linuxphoneapps.org/app-author/)) that a value get's replaced with a 0 character string.

The issues with that are:

1. Information that (likely) was carefully curated gets removed without a clear warning,
2. The taxonomy now contains an empty string, which breaks Zola building the page and thus makes the build pinepline fail.

How things should be:

Information should stay there, and the checker should instead give a warning to manually re-check the stored information, since the upstream appstream file does not contain it.