[BUGFIX] Prevent PHP errors / warnings on flexform resolving

This ensures that `pi_flexform` is actually set,
before accessing it and also only tries to convert
it, in case it's still a string.

Resolves: #105231
Releases: main, 12.4
Change-Id: If6210c9d23db97cac36f904984490e6bf0195ab7
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/86471


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>