Enable schedule_layer's end to be null in the JSON encoding

[shotarok]

What

• Enable schedule_layer's end to be null in the JSON encoding

Why

• end: null means the layer doesn't end in the endpoint schedules/{id}.
  
  from https://developer.pagerduty.com/api-reference/b3A6Mjc0ODE4NQ-update-a-schedule
• The end time of the schedule layer will remain even after removing pagerduty_schedule.layer.end PagerDuty/terraform-provider-pagerduty#451 is a related issue. The PagerDuty terraform provider doesn't send end: null even when pagerduty_schedule.layer.end was deleted.

[stmcallister]

Makes sense! Thank you! 🎉 👍 🌮