Issue #3613847: Fix phpstan: remove unnecessary ?? NULL coalesce operators (3.0.x backport)

Removes the ?? NULL operators that phpstan reports as nullCoalesce.unnecessary: the left operand can never be undefined, so the coalesce is dead code. No behavior change.

• domain/src/Form/DomainForm.php:98
• domain/src/Hook/DomainThemeHooks.php:50

The flagged expressions were read from the job output and confirmed with a local phpstan run per branch, rather than assumed: the set differs from branch to branch, and the module has other ?? NULL occurrences that guard possibly undefined array offsets and are correctly NOT flagged. After the change, a local run of the whole module reports no nullCoalesce.unnecessary and no new errors on this branch.