While working on T273776: Scale: ability to deploy without mentorship module, I realized that the default value of wgGEHomepageManualAssignmentMentorsList is null, while MentorPageMentorManager::getManuallyAssignedMentorsPage only recognizes '' as "no page provided", resulting in warnings logged on a default setup.
Let's make MentorPageMentorManager::getManuallyAssignedMentorsPage accept null as well.