🐛(backend) stream empty string with an async iterator under ASGI

In the content_retrieve action, if the document is not existing on the
object storage, we return an empty binary string. In the case the app is
ran as an ASGI application we still have a warning explaining it must
consume the iterator before sending it.

src/backend/core/api/viewsets.py

@@ -10,6 +10,7 @@ import logging
 import socket
 import uuid
 from collections import defaultdict
+from io import BytesIO
 from urllib.parse import unquote, urlencode, urlparse
 
 from django.conf import settings
@@ -35,6 +36,7 @@ import requests
 import rest_framework as drf
 import waffle
 from botocore.exceptions import ClientError
+from botocore.response import StreamingBody
 from csp.constants import NONE
 from csp.decorators import csp_update
 from lasuite.malware_detection import malware_detection
@@ -2143,7 +2145,9 @@ class DocumentViewSet(
                     return drf_response.Response(status=status.HTTP_304_NOT_MODIFIED)
                 case "NoSuchKey" | "404":
                     return StreamingHttpResponse(
-                        b"", content_type="text/plain", status=200
+                        content_stream(StreamingBody(BytesIO(b""), content_length=0)),
+                        content_type="text/plain",
+                        status=200,
                     )
                 case _:
                     raise