Add "ready->memory" to transitions in worker (#4728)

* Add "ready->memory" to transitions in worker

This should remove the error message in #4721 but is more of a band-aid
than a fix.  Ready->memory transitions shouldn't happen, but since they
do occassionally crop up, we might as well dispatch them appropriately
until the worker state machine is ready.

* Default to `no_value`

* Update distributed/worker.py

Co-authored-by: James Bourbeau <jrbourbeau@users.noreply.github.com>

Co-authored-by: James Bourbeau <jrbourbeau@users.noreply.github.com>

distributed/worker.py

@@ -466,6 +466,8 @@ def __init__(
             ("executing", "memory"): self.transition_executing_done,
             ("flight", "memory"): self.transition_flight_memory,
             ("flight", "fetch"): self.transition_flight_fetch,
+            # Shouldn't be a valid transition but happens nonetheless
+            ("ready", "memory"): self.transition_ready_memory,
             # Scheduler intercession (re-assignment)
             ("fetch", "waiting"): self.transition_fetch_waiting,
             ("flight", "waiting"): self.transition_flight_waiting,
@@ -1837,8 +1839,8 @@ def transition_ready_error(self, ts):
             assert ts.traceback is not None
         self.send_task_state_to_scheduler(ts)
 
-    def transition_ready_memory(self, ts, value=None):
-        if value:
+    def transition_ready_memory(self, ts, value=no_value):
+        if value is not no_value:
             self.put_key_in_memory(ts, value=value)
         self.send_task_state_to_scheduler(ts)