Ignore frequency rows with zero-value headways for frequencies_stop_t…

…imes (#3263)

warehouse/models/intermediate/gtfs/int_gtfs_schedule__frequencies_stop_times.sql

@@ -67,6 +67,7 @@ int_gtfs_schedule__frequencies_stop_times AS (
         WITH OFFSET AS iteration_num
     -- if end time_sec = headway_secs * [some integer] + start_time_sec, then we're not actually supposed to have a trip that starts at end_time_sec
     WHERE start_time_sec + iteration_num * headway_secs < end_time_sec
+    AND headway_secs > 0
 )
 
 SELECT * FROM int_gtfs_schedule__frequencies_stop_times