Change in `get_term()` return value with mismatched term ID and taxonomy

[dlh]

Before r34997, calling get_term() with a term ID and a taxonomy that were valid but mismatched would return null. That pairing now returns a WP_Error after get_term() calls _split_shared_term() to no effect.

To demonstrate, this test starts failing after r34997:

function test_get_term_return_value() {
    $term_id = $this->factory->term->create( array( 'taxonomy' => 'post_tag' ) );
    $this->assertNull( get_term( $term_id, 'category' ) );
}

get_term() already returned an error in some situations, so adding one here might not be worth worrying about. In my case, I happened to have a unit test that was not looking for a WP_Error and broke.

[boonebgorges]

Ah, thanks for the catch, @dlh. It should be fine to return null here. (I'd prefer to standardize on WP_Error, but I care little enough that preserving the previous behavior is probably preferable :) )

[boonebgorges]

In 35227:

Return null from get_term() on taxonomy mismatch.

[34997] caused get_term() to return an error object in the case when
$taxonomy did not match the taxonomy of the located term. This was an
inadvertant change from the previous behavior, when get_term() would return
null in these cases.

Props dlh.
See #14162. Fixes #34332.