Fix issue with default value for pymongo (#955)

Author: uriyyo

fastapi_pagination/ext/pymongo.py

@@ -28,7 +28,6 @@ def paginate(
     params, raw_params = verify_params(params, "limit-offset")
 
     query_filter = query_filter or {}
-    filter_fields = filter_fields or {}
 
     total = collection.count_documents(query_filter) if raw_params.include_total else None
     cursor = collection.find(