[BUGFIX] Filter empty values from language list

In case an old installation still has empty values stored in
`EXTCONF/lang/availableLanguages` (which has been fixed in #63546),
e.g. the language matrix in the "Manage Language Packs" is unusable.

This patch therefore removes empty values from the mentioned
configuration, as similar solved in the old issue.

Resolves: #89772
Related: #63546
Releases: master, 9.5
Change-Id: Id7473e5f17878d84914fec4df629ac6617256b55
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62497


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>