Fix PHP notice in WP_Tax_Query::get_sql_for_clause().

Previously, the `$where` variable was not initialized, so that when an invalid
'operator' was passed, PHP would complain that `$where` was undefined.

Props dlh.
Fixes #32756.

git-svn-id: https://develop.svn.wordpress.org/trunk@32910 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/taxonomy.php

@@ -1046,7 +1046,7 @@ class WP_Tax_Query {
 			'join'  => array(),
 		);
 
-		$join = '';
+		$join = $where = '';
 
 		$this->clean_query( $clause );