fix: make workflow read optional

[ghost]

To resolve:

3 validation errors for IncidentExpandedPagination
items.16.workflow_instances.0.workflow
  Field required [type=missing, input_value=<WorkflowInstance #36>, input_type=WorkflowInstance]
    For further information visit https://errors.pydantic.dev/2.11/v/missing
items.16.workflow_instances.1.workflow
  Field required [type=missing, input_value=<WorkflowInstance #37>, input_type=WorkflowInstance]
    For further information visit https://errors.pydantic.dev/2.11/v/missing
items.36.workflow_instances.0.workflow
  Field required [type=missing, input_value=<WorkflowInstance #38>, input_type=WorkflowInstance]
    For further information visit https://errors.pydantic.dev/2.11/v/missing