Don't let A & B be null when an empty array is passed

[niekp]

When passing an empty array to Diff the setSeq1 and setSeq2 check on null == [] will pass and a & b stay null.

This will cause an exception in getMatchingBlocks's count($this->a) on PHP 8.

In PHP 7.4 the count could handle null, but not anymore.
https://www.php.net/manual/en/function.count.php