[TASK] Streamline BackendLayout API class

Followup with minor corrections for https://review.typo3.org/c/Packages/TYPO3.CMS/+/63918

* Relaxes method signature of getSelectedBackendLayout
  to restore compatibilty with third party XCLASS.
* Avoids parsing empty configuration strings (in use
  cases where configuration is passed to BackendLayout
  as array, not TS string).
* Uses DrawingConfiguration instance provided from
  BackendLayout instead of creating a new instance to
  override the one returned from BackendLayout.
* Fixes a method call with unused return value.

Releases: master
Resolves: #90839
Change-Id: If6fe1286d3ce6530de30ea4c534ac1799e63177e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63968


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>