Add support for `'tag' => true` in `wp_kses_attr()`

[ocean90]

Related: ​[meta5483], #20017

wp_kses_attr() uses $allowed_html[strtolower($element)]) == 0 to check whether any attributes are allowed for an element. There's at least one place where true is used to define the support of no attributes. But because count( true ) returns 1 unnecessary checks for the support of attributes are performed.

I don't see anything wrong with using true as an alternative for an empty array and because it's already supported for attributes (#20017) I suggest to add official support for true in wp_kses_attr() as well.

[ocean90]

@dd32 Do you have any thoughts or objections?

[dd32]

I have no objections, seems like a sane thing to do (as I thought it already did work.. :))

[ocean90]

In 40637:

KSES: Support 'tag' => true as a shorthand for 'tag' => array() in wp_kses_attr().

Automatic_Upgrader_Skin::feedback() had always assumed that this is already the case, now it is.

See #20017.
Fixes #40680.