Malformed `object_types` when using `register_taxonomy_for_object_type()` on taxonomy that doesn't already have object types

[boonebgorges]

If you register a taxonomy with an empty $object_type, eg

register_taxonomy( 'foo', '' );

and later on decide to register the taxonomy with a post type:

register_taxonomy_for_object_type( 'foo', 'post' );

the 'object_types' property of the taxonomy object will be an array of the form:

array(
    '',
    'post',
)

This can cause various sorts of weirdness. The issue I ran into is that the 'update_count_callback' fallback ends up being _update_generic_term_count() instead of the correct _update_post_term_count(), because of the array_filter() check here: https://core.trac.wordpress.org/browser/tags/4.2.2/src/wp-includes/taxonomy.php?marks=3670-3676#L3657

[boonebgorges]

In 32709:

Filter out empty object_types in register_taxonomy_for_object_type().

This prevents weird edge bugs when registering an existing taxonomy with an
object type when the taxonomy was previously associated with no object types.

Fixes #32590.