Set password to empty string instead of None

Prevent exception in auth module.

radicale/__init__.py

@@ -338,7 +338,7 @@ class Application:
             user = self.Auth.map_login_to_user(login)
         else:
             user = self.Auth.map_login_to_user(environ.get("REMOTE_USER", ""))
-            password = None
+            password = ""
 
         # If "/.well-known" is not available, clients query "/"
         if path == "/.well-known" or path.startswith("/.well-known/"):