Assign s None instead of True

This works better if we want to type `s` as `set`. Also this is a more
typical default value when initializing a variable unlike `True`.

distributed/scheduler.py

@@ -2226,7 +2226,7 @@ async def add_worker(
             recommendations = {}
             for ts in list(self.unrunnable):
                 valid = self.valid_workers(ts)
-                if valid is True or ws in valid:
+                if valid is None or ws in valid:
                     recommendations[ts._key] = "waiting"
 
             if recommendations:
@@ -4657,7 +4657,7 @@ def decide_worker(self, ts: TaskState):
         """
         valid_workers = self.valid_workers(ts)
 
-        if not valid_workers and not ts._loose_restrictions and self.workers:
+        if valid_workers == set() and not ts._loose_restrictions and self.workers:
             self.unrunnable.add(ts)
             ts.state = "no-worker"
             return None
@@ -5518,14 +5518,14 @@ def check_idle_saturated(self, ws: WorkerState, occ: double = -1.0):
     def valid_workers(self, ts: TaskState):
         """Return set of currently valid workers for key
 
-        If all workers are valid then this returns ``True``.
+        If all workers are valid then this returns ``None``.
         This checks tracks the following state:
 
         *  worker_restrictions
         *  host_restrictions
         *  resource_restrictions
         """
-        s = True
+        s = None
 
         if ts._worker_restrictions:
             s = {w for w in ts._worker_restrictions if w in self.workers}
@@ -5539,7 +5539,7 @@ def valid_workers(self, ts: TaskState):
                 self.host_info[h]["addresses"] for h in hr if h in self.host_info
             ]
             ss: set = set.union(*sl) if sl else set()
-            if s is True:
+            if s is None:
                 s = ss
             else:
                 s |= ss
@@ -5555,12 +5555,12 @@ def valid_workers(self, ts: TaskState):
             }
 
             ww: set = set.intersection(*dw.values())
-            if s is True:
+            if s is None:
                 s = ww
             else:
                 s &= ww
 
-        if s is True:
+        if s is None:
             return s
         else:
             return {self.workers[w] for w in s}