[BUGFIX] Handle empty tags in language pack index files

This patch adds a check to the L10n index parser to consider
whether the value of an XML tag is empty or contains only
spaces. In this case it is not included in the resulting array.

Resolves: #41450
Releases: 6.2, 6.1, 6.0
Change-Id: Ia17a430f3ab2d79e2f97b55feb9d3c4ee60bfb7e
Reviewed-on: https://review.typo3.org/15138
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind