Revert dictionary parameter back to optional

This reverts commit 52d352d.
See: #870 (comment)

Author: ellnix

meilisearch/index.py

@@ -1594,7 +1594,7 @@ def get_dictionary(self) -> List[str]:
         """
         return self.http.get(self.__settings_url_for(self.config.paths.dictionary))
 
-    def update_dictionary(self, body: List[str]) -> TaskInfo:
+    def update_dictionary(self, body: Union[List[str], None]) -> TaskInfo:
         """Update the dictionary of the index.
 
         Parameters