extra headers must be a dictionary

a missing path would cause http handler errors

xpra/net/http/directory_listing.py

@@ -18,7 +18,7 @@ def list_directory(path:str):
     try:
         dirlist = os.listdir(path)
     except OSError:
-        return 404, None, b"No permission to list directory"
+        return 404, {}, b"No permission to list directory"
     dirlist.sort(key=lambda a: a.lower())
     r = []
     try: