Fix for API modified_after (case where last_modified is null)
Fix for API modified_after (case where last_modified is null)

kickapoo/enhydris,openmeteo/enhydris,openmeteo/enhydris,ellak-monades-aristeias/enhydris,openmeteo/enhydris,aptiko/enhydris,ellak-monades-aristeias/enhydris,ellak-monades-aristeias/enhydris,kickapoo/enhydris,kickapoo/enhydris,aptiko/enhydris,aptiko/enhydris