[CSGI-2016] Address pagerduty_schedule validation error on weekly restriction wihtout Start of week day

[imjaroiswebdev]

When a pagerduty_schedule layer is configured with a weekly_restriction type, without setting a start_day_of_week a validation error will be returned during planning stage, like the following...

╷
│ Error: start_day_of_week must be set for a weekly_restriction schedule restriction type
│

Test case extended...

$ make testacc TESTARGS="-run TestAccPagerDutySchedule_Basic"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run TestAccPagerDutySchedule_Basic -timeout 120m
?       github.com/terraform-providers/terraform-provider-pagerduty     [no test files]
=== RUN   TestAccPagerDutySchedule_Basic # 👈 Test case extended
--- PASS: TestAccPagerDutySchedule_Basic (28.08s)
=== RUN   TestAccPagerDutySchedule_BasicWithExternalDestroyHandling
--- PASS: TestAccPagerDutySchedule_BasicWithExternalDestroyHandling (15.25s)
=== RUN   TestAccPagerDutySchedule_BasicWeek
--- PASS: TestAccPagerDutySchedule_BasicWeek (17.00s)
PASS
ok      github.com/terraform-providers/terraform-provider-pagerduty/pagerduty   61.092s