PHP 7.2: Skip counting currentNesting if null in Injector

[zerocrates]

This is an error starting in PHP 7.2, causing a test failure:

Unexpected PHP Error [count(): Parameter must be an array or an object that implements Countable] severity [2] in [HtmlPurifier/library/HTMLPurifier/Injector.php line 160]
in testErrorRequiredElementNotAllowed
in HTMLPurifier_Strategy_MakeWellFormed_InjectorTest

I used empty to parallel an existing test in the same method.