[BUGFIX] Do not try to parse null/empty flexforms in ReferenceIndex

This fixes an issue in the acceptance tests where `null` is passed
to `GeneralUtility::xml2array()` (which expects a `string`).

Having an early check for an empty string also avoids
pointless XML parsing (which should help performance a bit).

Resolves: #101757
Releases: main, 12.4, 11.5
Change-Id: Ifc4406536c273baca9acdda7a0af7a91594f57bf
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80680


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>