[BUGFIX] Function menu: Removed call to deprecated method render()

Call of FlashMessage->render() if no modules have been registered
was replaced with appropriate code.

Resolves: #73221
Releases: master, 7.6
Change-Id: I982e2d7d5b677e77a7f170a505f426780ac77089
Reviewed-on: https://review.typo3.org/46581


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>