Have UserDict.__init__() implicitly check for updating w/ bool(kwargs) instead of len() (GH-12139)

Semantically the same, but more idiomatic by checking against `kwargs` instead of `len(kwargs)`.

Lib/collections/__init__.py

@@ -1016,7 +1016,7 @@ class UserDict(_collections_abc.MutableMapping):
         self.data = {}
         if dict is not None:
             self.update(dict)
-        if len(kwargs):
+        if kwargs:
             self.update(kwargs)
     def __len__(self): return len(self.data)
     def __getitem__(self, key):