Make MapMutation.update consistently return an implicit None

Author: hukkinj1

immutables/map.py

@@ -740,8 +740,7 @@ def update(self, col=None, **kw):
                 it = iter(kw.items())
 
         if it is None:
-
-            return self
+            return
 
         root = self.__root
         count = self.__count