[BUGFIX] Queries with NULL don't work anymore

See change: I517c0500cad75cae96fd739f2ff7222db80b42a5
At the end of the Typo3DbQueryParser::parseComparison() method the updated
operator for NULL values was no longer passed to the parseDynamicOperand,
generating = NULL instead of IS NULL.

Solved the issue by adding the operator parameter again.

Change-Id: I13c3cecf0bf4c95789032f31ead192b945794484
Resolves: #57130
Related: #55167
Releases: 6.2
Reviewed-on: https://review.typo3.org/28592
Reviewed-by: Markus Klein
Reviewed-by: Felix Oertel
Tested-by: Felix Oertel