Add return type

server/vbv_lernwelt/dashboard/person_export.py

@@ -1,4 +1,5 @@
 from io import BytesIO
+from typing import Optional
 
 import structlog
 from django.utils.translation import gettext_lazy as _
@@ -22,8 +23,8 @@ def export_persons(
     user: User,
     course_session_ids: list[str],
     save_as_file: bool = False,
-):
-    if len(course_session_ids) == 0:
+) -> Optional[bytes]:
+    if not course_session_ids:
         return
 
     wb = Workbook()