[TASK] Remove unselected languages in LocalConfiguration

If selecting one ore more languages in EXT:lang, the complete
language tree is written in EXTCONF/lang/availableLanguages in
LocalConfiguration.php. Empty values in this array represent
unselected languages. This patch removes the empty values
from the language array.

Resolves: #63546
Releases: master, 6.2
Change-Id: I327f05c16f3824f1c91c8f2e095c5b5059b420b9
Reviewed-on: http://review.typo3.org/35011
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>