ApiBlock: Avoid PHP warning when partial blocks are enabled but not used

ApiBlock: Avoid PHP warning when partial blocks are enabled but not used

If partial blocks are available but a sitewide block is being made,
$params['pagerestrictions'] is null and implode() raises a warning.
Since null casts to the empty array, it's easy enough to work around.

Also add a test hitting this case.

Change-Id: Id7e2559d7569031b7c1228adb0c0a14b3c1527c3