REST API: Allow sending empty array to remove all values for single = false meta

[chrisvanpatten]

When sending an update to a piece of content via the REST API, meta registered with 'single' => false cannot be removed by sending an empty array; you must send null.

The inability to send an empty array is slightly confusing and inconsistent with array-type meta registered with 'single' => true, which will correctly save the empty value.

[TimothyBlynJacobs]

@chrisvanpatten Do you want to work on a patch for this? We have a bit more time for inclusion in 5.6.

I think the fix would be updating the is_null() clause in WP_REST_Meta_Fields::update_value() to a accept an empty array if the meta type is not single.

Allow passing an empty array to the REST API for registered non-single meta to remove the DB values, as an alternative to sending null.

Trac ticket: https://core.trac.wordpress.org/ticket/50790

[TimothyBlynJacobs]

In 49966:

REST API: Allow sending an empty array to delete multi meta keys.

Previously, only null was supported.

Fixes #50790.
Props chrisvanpatten.

Merged in ​https://github.com/WordPress/wordpress-develop/commit/244e4d4e637faa90d24a830690671064bb3fb48a.