[BUGFIX] Restore contentObject condition in EXT:form

With #86196 the condition for `contentObject` has been
accidentally removed.

Resolves: #92471
Related: #86196
Releases: main, 11.5
Change-Id: I2c6ca4668141639650e3b4b1b58da2270f827f76
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73654


Tested-by: core-ci <typo3@b13.com>
Tested-by: waldhacker <hello@waldhacker.dev>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: waldhacker <hello@waldhacker.dev>