Code Modernization: Return false from wpdb::query() if the query was filtered to an empty string using the query filter.

This avoids a fatal error on PHP 8 caused by passing an empty string to `mysqli_query()`, and maintains the current behaviour.

Follow-up to [48980], [48981].

See #50913, #50639.
Built from https://develop.svn.wordpress.org/trunk@49072


git-svn-id: http://core.svn.wordpress.org/trunk@48834 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/wp-db.php

@@ -1915,6 +1915,11 @@ class wpdb {
 		 */
 		$query = apply_filters( 'query', $query );
 
+		if ( ! $query ) {
+			$this->insert_id = 0;
+			return false;
+		}
+
 		$this->flush();
 
 		// Log how the function was called.