[BUGFIX] Only render widget footer if defined and correct styling issues

Widget footer is now only rendered if the optional footer section for
widgets is used and contains content. In addition, minor styling issues
were resolved. Templates were cleaned up again. Some extra conditions
were added to prevent empty tags.

Resolves: #90530
Releases: master
Change-Id: Idb4a6858395622a62448041fec3d510bec6fb271
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63410


Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>