SpecialUnblock: Allow getTargetAndType to accept null $par
Also add documentation and type hinting for
SpecialBlock::getTargetAndType
Bug: T263642
Change-Id: If3e78537cd813d8aabbc6ba53c4d0be949ede9a1
(cherry picked from commit 7432a7c7ad8e1ff077e9e20f19279057f139c40d)