chore: make mypy happy

Author: pawel-kmiecik

src/unstructured_client/_hooks/custom/clean_server_url_hook.py

@@ -10,11 +10,11 @@
 class CleanServerUrlSDKInitHook(SDKInitHook):
     """Hook fixing common mistakes by users in defining `server_url` in the unstructured-client"""
 
-    def clean_server_url(self, base_url) -> str:
+    def clean_server_url(self, base_url: str) -> str:
         """Fix url scheme and remove the '/general/v0/general' path."""
 
         if not base_url:
-            return None
+            return ""
         # -- add a url scheme if not present (urllib.parse does not work reliably without it)
         if "http" not in base_url:
             base_url = "http://" + base_url