Ability to display schedule and current oncall without escalation policy

[iksaif]

Is your feature request related to a problem? Please describe.
Currently when clicking on a schedule the widget will be empty unless the schedule has an associated escalation policy.

Describe the solution you'd like
It would be nice for it to work without escalation policy

[kathy-huang]

To add onto this, we've got some schedules not associated with escalation policies, so these users aren't listed in the /oncalls endpoint. Instead, these schedules are in the /schedules endpoint under final_schedule.rendered_schedule_entries .