Unnecessary update when value is None

When value is None, it is not necessary to invoke update and run other methods and conditions to make the code complicated there.

scrapy/settings/__init__.py

@@ -83,7 +83,8 @@ class BaseSettings(MutableMapping):
     def __init__(self, values=None, priority='project'):
         self.frozen = False
         self.attributes = {}
-        self.update(values, priority)
+        if values is not None:
+            self.update(values, priority)
 
     def __getitem__(self, opt_name):
         if opt_name not in self: