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

snikket_web/user.py

@@ -1,4 +1,3 @@
-import aiohttp
 import asyncio
 import typing
 import urllib
@@ -188,16 +187,12 @@ async def manage_data() -> typing.Union[str, quart.Response]:
         encoded_address = urllib.parse.quote(
             user_info["address"].encode(encoding='utf-8', errors='strict')
         )
-        try:
-            account_data = await client.export_account_data()
-        except aiohttp.ClientResponseError as e:
-            if e.status == 404:
-                await flash(
-                    _("You currently have no account data to export."),
-                    "alert"
-                )
-            else:
-                raise e
+        account_data = await client.export_account_data()
+        if account_data is None:
+            await flash(
+                _("You currently have no account data to export."),
+                "alert"
+            )
         else:
             return Response(account_data,
                             mimetype="application/xml",