Code Modernization: Fix "passing null to non-nullable" deprecation no…

…tice in `WP_Comment_Query::get_comment_ids()`.

The `WP_Comment_Query::get_comment_ids()` method is supposed to handle `null` as a search query, but was throwing a `strlen(): Passing null to parameter #1 ($string) of type string is deprecated` notice on PHP 8.1.

Discovered via and already covered via the pre-existing `Tests_Comment_Query::test_search_null_should_be_ignored()` test method.

Follow-up to [36345], [48275].

Props jrf, hellofromTonya.
See #53635.

git-svn-id: https://develop.svn.wordpress.org/trunk@51806 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-comment-query.php

@@ -801,7 +801,7 @@ protected function get_comment_ids() {
 		}
 
 		// Falsey search strings are ignored.
-		if ( strlen( $this->query_vars['search'] ) ) {
+		if ( isset( $this->query_vars['search'] ) && strlen( $this->query_vars['search'] ) ) {
 			$search_sql = $this->get_search_sql(
 				$this->query_vars['search'],
 				array( 'comment_author', 'comment_author_email', 'comment_author_url', 'comment_author_IP', 'comment_content' )