[BUGFIX] Harden MigrateFeloginPlugins for empty flexforms

Avoid the migration of empty flexforms, which caused the wizard to fail.

Resolves: #97761
Releases: 11.5
Change-Id: I9240becb81ddca22432e8ba55038978f30fcc449
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76529


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