`wp_update_term()` doesn't check for `null` after `get_term()`

[dlh]

get_term() can return null, but wp_update_term() checks $term for only a WP_Error before proceeding as if it has an array. This can lead to a warning in array_merge($term, $args) and a notice in wp_unslash( $args['name'] ).

The attached patch adds a check for null and a unit test.

[SergeyBiryukov]

We could use an existing string, 'Empty Term' (upper case).

[SergeyBiryukov]

In 32117:

wp_update_term() should check if get_term() returned null.

props dlh.
fixes #31954.