[BUGFIX] Prevent warning when FlexForm is empty in PHP8

When a FlexForm has an empty data structure, a warning is thrown
in PHP8, because it is not checked in FlexFormNoTabsContainer if
the resolved sheet name actually exists as key in the
flexFormRowData array.

This patch uses the null coalescing operator to fix the problem.

Resolves: #98147
Releases: main, 11.5
Change-Id: Ie4bde3af79a9a473cc02aff9f1ec299fff1bc259
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75495


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