Ensure that the `$exclusions` parameter of 'list_terms_exclusions' filter is always a string.

Props fhwebcs.
Fixes #31681.

git-svn-id: https://develop.svn.wordpress.org/trunk@31813 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/taxonomy.php

@@ -1824,6 +1824,8 @@ function get_terms( $taxonomies, $args = '' ) {
 
 	if ( ! empty( $exclusions ) ) {
 		$exclusions = ' AND t.term_id NOT IN (' . implode( ',', array_map( 'intval', $exclusions ) ) . ')';
+	} else {
+		$exclusions = '';
 	}
 
 	/**