Let Langugae.use_params work with falsey inputs

The Language.use_params method was failing if you passed in None, which
meant we had to use awkward conditionals for the parameter averaging.
This solves the problem.

spacy/language.py

@@ -1,5 +1,5 @@
 from typing import Optional, Any, Dict, Callable, Iterable, Union, List, Pattern
-from typing import Tuple, Iterator
+from typing import Tuple, Iterator, Optional
 from dataclasses import dataclass
 import random
 import itertools
@@ -1275,7 +1275,7 @@ class Language:
         return results
 
     @contextmanager
-    def use_params(self, params: dict):
+    def use_params(self, params: Optional[dict]):
         """Replace weights of models in the pipeline with those provided in the
         params dictionary. Can be used as a contextmanager, in which case,
         models go back to their original weights after the block.
@@ -1288,6 +1288,9 @@ class Language:
 
         DOCS: https://spacy.io/api/language#use_params
         """
+        if not params:
+            yield
+        else:
             contexts = [
                 pipe.use_params(params)
                 for name, pipe in self.pipeline