Remove is_null() checks in apply_filters(), do_action(), et al.

[nacin]

In apply_filters(), do_action(), and friends, there is a check for ! is_null( $the['function'] ). As these functions get called collectively thousands of times per page, a tiny check like this can add up.

A few months ago, I decided to track it down. They were shuffled around a bit in [4955], but they actually had roots in [1394]. The link there (​http://www.kackreiz.net/wordpress.php) is dead, but I found it at ​http://www.kackreiz.net/wordpress/apply_filters.html#fixed, via ​http://wordpress.org/support/topic/bug-in-remove_filterapply_filters?replies=3.

The original bug was that remove_filter() left NULL's in the $wp_filter array, rather than properly unsetting it as it was modified to do long ago. Now that we no longer leave NULL scattered about, this check is safe to remove.

[nacin]

This requires either #21169 or a change to remove_filter().

[nacin]

In [21287]:

Remove unnecessary is_null() checks from the worker loops inside do_action(), apply_filters(), etc. fixes #21321. see #21169.

[nacin]

In [21299]:

Revert [21287] until #21169 goes through. see #21321.

[nacin]

Some side effects here, due to our janky current() loop and self-removal of actions. see ​[UT935]

[DrewAPicture]

@nacin: Do we still need to handle this?

[johnbillion]

This might be mooted by #17817, but FWIW null !== $x is marginally faster than ! is_null( $x ). On 10,000 iterations you save around 200ms.