* (bug 29116) Fix regression breaking CheckUser extension

* (bug 29116) Fix regression breaking CheckUser extension

Fixes regression from r84475 and friends which made Block->load() and its new front-end Block::newFromTarget() fail when an empty string was passed in as the IP / $vagueTarget parameter to indicate skipping IP-based lookups.
Added phpunit test cases to confirm that both Block->load() and Block::newFromTarget() work when given null (already ok), '' (as done from CheckUser), or false (not seen, but perfectly legit sounding).
Adjusted comparisons to work as expected.