Fix zero length media handling (#17570)

Results in:

```
AssertionError: null
  File "synapse/http/server.py", line 332, in _async_render_wrapper
    callback_return = await self._async_render(request)
  File "synapse/http/server.py", line 544, in _async_render
    callback_return = await raw_callback_return
  File "synapse/federation/transport/server/_base.py", line 369, in new_func
    response = await func(
  File "synapse/federation/transport/server/federation.py", line 826, in on_GET
    await self.media_repo.get_local_media(
  File "synapse/media/media_repository.py", line 473, in get_local_media
    await respond_with_multipart_responder(
  File "synapse/media/_base.py", line 353, in respond_with_multipart_responder
    assert content_length is not None
```

changelog.d/17570.bugfix

@@ -0,0 +1 @@
+Fix bug where we would respond with an error when a remote server asked for media that had a length of 0, using the new multipart federation media endpoint.

synapse/media/media_storage.py

@@ -544,7 +544,7 @@ def content_length(self) -> Optional[int]:
         Calculate the content length of the multipart response
         in bytes.
         """
-        if not self.length:
+        if self.length is None:
             return None
         # calculate length of json field and content-type, disposition headers
         json_field = json.dumps(self.json_field)