[BUGFIX] Avoid undefined array key "templateName" in FormEditorController

Use null coalescing operator to guard against
undefined array key "templateName" accesss in
`\TYPO3\CMS\Form\Controller\FormEditorController`.

Resolves: #99870
Releases: main, 11.5
Change-Id: I3ebc40e6caab8c2f5bb32563848e836171c44f7b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77785


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>