Avoid panic at null workflow in IW trigger, allowing it to refresh

[cjgajard]

=== RUN   TestAccPagerDutyIncidentWorkflowTrigger_import
--- PASS: TestAccPagerDutyIncidentWorkflowTrigger_import (17.80s)
=== RUN   TestAccPagerDutyIncidentWorkflowTrigger_BadType
--- PASS: TestAccPagerDutyIncidentWorkflowTrigger_BadType (0.20s)
=== RUN   TestAccPagerDutyIncidentWorkflowTrigger_ConditionWithManualType
--- PASS: TestAccPagerDutyIncidentWorkflowTrigger_ConditionWithManualType (0.17s)
=== RUN   TestAccPagerDutyIncidentWorkflowTrigger_ConditionalTypeWithoutCondition
--- PASS: TestAccPagerDutyIncidentWorkflowTrigger_ConditionalTypeWithoutCondition (0.17s)
=== RUN   TestAccPagerDutyIncidentWorkflowTrigger_SubscribedToAllWithInvalidServices
--- PASS: TestAccPagerDutyIncidentWorkflowTrigger_SubscribedToAllWithInvalidServices (0.17s)
=== RUN   TestAccPagerDutyIncidentWorkflowTrigger_BasicManual
--- PASS: TestAccPagerDutyIncidentWorkflowTrigger_BasicManual (15.62s)
=== RUN   TestAccPagerDutyIncidentWorkflowTrigger_BasicConditionalAllServices
--- PASS: TestAccPagerDutyIncidentWorkflowTrigger_BasicConditionalAllServices (12.91s)
=== RUN   TestAccPagerDutyIncidentWorkflowTrigger_ManualWithTeamPermissions
--- PASS: TestAccPagerDutyIncidentWorkflowTrigger_ManualWithTeamPermissions (29.11s)
=== RUN   TestAccPagerDutyIncidentWorkflowTrigger_ChangeTypeCausesReplace
--- PASS: TestAccPagerDutyIncidentWorkflowTrigger_ChangeTypeCausesReplace (12.59s)
=== RUN   TestAccPagerDutyIncidentWorkflowTrigger_CannotChangeType
--- PASS: TestAccPagerDutyIncidentWorkflowTrigger_CannotChangeType (12.51s)
PASS
ok      github.com/PagerDuty/terraform-provider-pagerduty/pagerduty     101.762s