Evict keys when they're set to None in PermissionOverwrite

Fixes #5929

discord/permissions.py

@@ -484,6 +484,9 @@ class PermissionOverwrite:
         if value not in (True, None, False):
             raise TypeError('Expected bool or NoneType, received {0.__class__.__name__}'.format(value))
 
+        if value is None:
+            self._values.pop(key, None)
+        else:
             self._values[key] = value
 
     def pair(self):
@@ -525,7 +528,7 @@ class PermissionOverwrite:
         :class:`bool`
             Indicates if the overwrite is empty.
         """
-        return all(x is None for x in self._values.values())
+        return len(self._values) == 0
 
     def update(self, **kwargs):
         r"""Bulk updates this permission overwrite object.