#3451214 Fix "mb_strtolower(): Passing null to parameter #1 ($string) of type string is deprecated" warning

aimairequested to merge
issue/block_exclude_pages-3555253:3555253-deprecated-function-mbstrtolower into 2.2.x

Issue #3555253 by aimai: Fix "mb_strtolower(): Passing null to parameter #1 ($string) of type string is deprecated" warning

Closes #3555253

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