🐛 Handle non-existing user IDs in read_user_by_id (#1396)

Co-authored-by: Motov Yurii <109919500+YuriiMotov@users.noreply.github.com>

backend/app/api/routes/users.py

@@ -170,6 +170,8 @@ def read_user_by_id(
             status_code=403,
             detail="The user doesn't have enough privileges",
         )
+    if user is None:
+        raise HTTPException(status_code=404, detail="User not found")
     return user