Switch to HTTP 204 to indicate no data to export

This is more robust, as it indicates the request was successfully
authenticated and processed, but that there is no data to export. This is
different from the URL not existing (which would also happen if the module was
unavailable, which should be a notable error instead).

snikket_web/prosodyclient.py

@@ -1129,11 +1129,13 @@ class ProsodyClient:
             self,
             *,
             session: aiohttp.ClientSession,
-            ) -> str:
+            ) -> typing.Optional[str]:
         async with session.get(
                 self._xep227_endpoint("/export?stores=roster,vcard,pep"),
                 ) as resp:
             self._raise_error_from_response(resp)
+            if resp.status == 204:
+                return None
             return await resp.text()
 
     @autosession