[jobs] allow for null schedule_state (skypilot-org#4588)

* [jobs] allow for null schedule_state

* Update sky/jobs/state.py

Co-authored-by: Zhanghao Wu <[email protected]>

* add to docstring

---------

Co-authored-by: Zhanghao Wu <[email protected]>

sky/jobs/state.py

@@ -313,6 +313,8 @@ def failure_statuses(cls) -> List['ManagedJobStatus']:
 class ManagedJobScheduleState(enum.Enum):
     """Captures the state of the job from the scheduler's perspective.
 
+    A job that predates the introduction of the scheduler will be INVALID.
+
     A newly created job will be INACTIVE.  The following transitions are valid:
     - INACTIVE -> WAITING: The job is "submitted" to the scheduler, and its job
       controller can be started.
@@ -342,6 +344,10 @@ class ManagedJobScheduleState(enum.Enum):
     briefly observe inconsistent states, like a job that just finished but
     hasn't yet transitioned to DONE.
     """
+    # This job may have been created before scheduler was introduced in #4458.
+    # This state is not used by scheduler but just for backward compatibility.
+    # TODO(cooperc): remove this in v0.11.0
+    INVALID = None
     # The job should be ignored by the scheduler.
     INACTIVE = 'INACTIVE'
     # The job is waiting to transition to LAUNCHING for the first time. The