Extra protection for empty order key orders.

Some order can have order key set to empty string. This commit disallows fetching those orders via key at DB level (its already disallowed from interface).

plugins/woocommerce/src/Internal/DataStores/Orders/OrdersTableDataStore.php

@@ -847,7 +847,7 @@ WHERE
 			$wpdb->prepare(
 				"SELECT {$orders_table}.id FROM {$orders_table}
 				INNER JOIN {$op_table} ON {$op_table}.order_id = {$orders_table}.id
-				WHERE {$op_table}.order_key = %s",
+				WHERE {$op_table}.order_key = %s AND {$op_table}.order_key != ''",
 				$order_key
 			)
 		);