parse_request() of WP should set empty query variables when appropriate

[filosofo]

As ​scribu points out, if a custom query variable is empty it remains unset.

Patch changes the empty check on permalink variables to an isset check.

This shouldn't affect core query variables, because it seems as though they're eventually all set even when empty.

[scribu]

Related #11330

[filosofo]

Another reason for this is that when endpoint rewrite rules are generated, they explicitly allow for empty endpoint values (see line 1289ff of wp-includes/rewrite.php).

So WP rewrite is writing checks that WP query can't cash.

[scribu]

(In [15770]) Make parse_reques() set empty variables. Props filosofo for initial patch. Fixes #10710

[scribu]

PS: I have no idea why elseif ( isset( $GLOBALS[$wpvar] ) ) is there.

[hakre]

Replying to scribu:

PS: I have no idea why elseif ( isset( $GLOBALS[$wpvar] ) ) is there.

That's a feature so that plugins can re-enable register globals functionality.

[scribu]

Thanks for the info.

[scribu]

Related: #15112

[ryan]

This causes #16002