[BUGFIX] Check for children in FormEngine container

When resolving the first and last inline
elements in the `FilesControlContainer`
and `InlineControlContainer`, it is now
properly checked, whether any children
exist at all, preventing possible type
errors.

Additionally a fallback is added to
prevent further warnings in the containers
while trying to iterate over the children.

Resolves: #103646
Releases: main, 12.4
Change-Id: I6f46ee5dc6d5553f3bd43c12447ef2390e1f941f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83895


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Oliver Bartsch <bo@cedev.de>