[BUGFIX] Suppress empty tag names in output of array2xml

If a tag name is empty GeneralUtility::array2xml must not output that
tag at all.

Resolves: #52529
Releases: 6.2, 6.1, 6.0
Change-Id: Ib075feac25e5ba06884436db6d4be69cc428a43d
Reviewed-on: https://review.typo3.org/24849
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert