[BUGFIX] Fix PHP warning in FormFieldViewHelper

When using a formField with a property name
that only consists of a number (e.g. a UID),
Fluid will now allow this.

Resolves: #100281
Releases: main, 11.5
Change-Id: Ief95b87af1025f65ac0db1ed0c64b6fcf9c40bbe
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78573


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