[BUGFIX] Make empty $GLOBALS['TBE_MODULES_EXT'] traversable

Prevent warnings and notices if $GLOBALS['TBE_MODULES_EXT'] is not
set, which can be the case if no extensions are installed.

Change-Id: I72f513329790ee3b71fd30054574b79f41a6dcf8
Resolves: #83479
Releases: master, 8.7, 7.6
Reviewed-on: https://review.typo3.org/55297


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Richard Haeser <richard@maxserv.com>
Tested-by: Richard Haeser <richard@maxserv.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>