[BUGFIX] Fix uninitialized $queuedMessages in MemorySpool

The protected member $queuedMessages of MemorySpool is now
initialized as empty array to prevent exceptions on accessing
it in magic methods like __toString().

Resolves: #92135
Releases: master, 10.4
Change-Id: I2e12c52be02caa82e3fbdb1014433709589b9752
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/65804


Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Alexander Schnitzler <git@alexanderschnitzler.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Alexander Schnitzler <git@alexanderschnitzler.de>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>