WP_Query::tax_query property could be null

[manooweb]

By verifying a third party plugin code with PHPStan tool and WordPress stubs, it occurs that in some context WP_Query::tax_query property could be null.

The joined patch propose to enhance the WP_Query::tax_query documentation.

[manooweb]

WP_Query::tax_query doc enhancement

[audrasjb]

Thanks!
Self assigning for final check and commit.

[audrasjb]

@manooweb quick question: do you have any example of a situation where tax_query could be null?

[manooweb]

Replying to audrasjb:

@manooweb quick question: do you have any example of a situation where tax_query could be null?

It isn't so easy to reply.
If I'm not mistaken, I saw tax_query is set only in some conditions by calling WP_Query::parse_tax_query() in WP_Query::parse_query() or WP_Query::get_posts(). So it's possible that this property isn't set and instantiate because WP_Query::parse_tax_query() isn't executed.
​https://github.com/WordPress/WordPress/blob/6.4.3/wp-includes/class-wp-query.php#L1372

It's simply a documentation proposition.
When we try to access to the global variable $wp_query (or $wp_the_query) we have no chance to know the tax_query property has been set yet because this property isn't set by the WP_Query instantiation but during the WP_Query::get_posts() execution.

I hope it's clearer.

[audrasjb]

In 57761:

Docs: Update WP_Query::tax_query docblock to reflect that the property can be null.

Props manooweb, swissspidy.
Fixes #60563.