wp.editPost does not allow removal of all terms for a taxonomy

[maxcutler]

wp.newPost and wp.editPost XML-RPC methods have a terms argument which allows setting of the terms associated with the post. This argument is an array with taxonomy names as keys and arrays of term IDs as values.

If a taxonomy is explicitly supplied with an empty list of IDs, the expectation is that this would cause removal of all currently-associated terms in that taxonomy from the post.

For example, if a post has 'Cat A' and 'Cat B' associated with it, passing array( 'category' => array() ) should cause both of those categories to be unassociated with the post.

This issue has existed since I wrote this code back in 3.4.

[maxcutler]

Added patch with unit test.

[SergeyBiryukov]

Duplicate of #26686.