fix(files): CR: add explicit comparison to None

Co-authored-by: Nejc Habjan <hab.nejc@gmail.com>

gitlab/v4/objects/files.py

@@ -267,7 +267,7 @@ def raw(
         """
         file_path = utils.EncodedId(file_path)
         path = f"{self.path}/{file_path}/raw"
-        if ref:
+        if ref is not None:
             query_data = {"ref": ref}
         else:
             query_data = None