Make some strlen() calls in OAuth code safe for PHP 8.0

Aklapper requested to merge T343923strlenPhp81 into wmf/stable

Since PHP 8.0, passing null to strlen() is deprecated. Thus instead check if a variable (whether string or not) is null or empty if we do not care about the actual byte length of a string anyway.

Bug: T343568

Bug: T343840

Edited by Aklapper

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