[BUGFIX] PHP 8.0 warnings in QueryGenerator

Yet another series of PHP 8 stabilizations for
ext:lowlevel QueryGenerator. This time found by
functional tests when error handling in functionals
is turned off.

* Various null coalescence within class itself

* A test setup change which string_replace()'ed
  within a serialized string, which invalidates
  length specifications. Fixed by serializing
  after replace operations, instead of before.

Resolves: #95562
Releases: master
Change-Id: Id0b06f1059bfb39296ccdb9cc37ff610652ba3f5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71531


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