Guard against null child node in read-only detection

Author: JanJakes

packages/mysql-on-sqlite/src/sqlite/class-wp-pdo-mysql-on-sqlite.php

@@ -896,8 +896,8 @@ public function query( string $query, ?int $fetch_mode = null, ...$fetch_mode_ar
 			 * [GRAMMAR]
 			 * simpleStatement: selectStatement | showStatement | utilityStatement | ...
 			 */
-			$statement_node = $child_node->get_first_child_node();
-			if ( null !== $statement_node ) {
+			if ( null !== $child_node && $child_node->has_child_node() ) {
+				$statement_node = $child_node->get_first_child_node();
 				if (
 					'selectStatement' === $statement_node->rule_name
 					|| 'showStatement' === $statement_node->rule_name