[BUGFIX] Hide empty tabs in DynTabMenu

Due to a previous tab patch, empty
tabs in e.g. FormEngine are always
shown but disabled (= unstyled).

The patch sets the default tab
behaviour to hide tabs that are
empty.

Releases: master
Resolves: #62972
Change-Id: I1d9c99269204cfdefa67b74085118def22b1ff9f
Reviewed-on: http://review.typo3.org/34198
Reviewed-by: Felix Kopp <felix-source@phorax.com>
Tested-by: Felix Kopp <felix-source@phorax.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>