Type error in wp_xmlrpc_server::_prepare_taxonomy()

[johnbillion]

There's a type error in the wp_xmlrpc_server::_prepare_taxonomy() method.

Ref: https://core.trac.wordpress.org/browser/tags/5.5.1/src/wp-includes/class-wp-xmlrpc-server.php?marks=768-770#L742

If $fields array contains a menu element then it tries to read the show_in_menu property from $_taxonomy, but this is an array not an object.

Probably just needs $_taxonomy changed to $taxonomy but needs testing, ideally needs a unit test and steps to reproduce.

Introduced in [25133] / #20930.

[johnbillion]

In 49856:

XML-RPC: Fix a type error when requesting the menu field of a taxonomy.

Fixes #51493