[BUGFIX] Fluid: Strict type checking when adding debug namespace

The check on BE_USER->uc() in frontend context is cleaned up and more
strict, checking whether the object exists currently.

Additionally, the specific request type is used here.

Resolves: #82397
Releases: master, 8.7
Change-Id: Ibe0b533c23e1ba9c35ed2b6b9b0301e1a449c2e3
Reviewed-on: https://review.typo3.org/53832


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>