User primary group deselect
Should admin be able to unset primary groups for users? 

Currently attempting to select the "none" option for a user already assigned a primary group results in an `PATCH http://localhost:9950/api/users/a1 400 (Bad Request)` error with a `{ "id": "bad_request", "message": "Primary group does not exist"}` response.
diff --git a/virtool/api/users.py b/virtool/api/users.py
index 6195bc693..c791e1b46 100644
--- a/virtool/api/users.py
+++ b/virtool/api/users.py
@@ -132,7 +132,9 @@ async def edit(req):
         if missing:
             return bad_request("Groups do not exist: " + ", ".join(missing))
 
-    if "primary_group" in data and data["primary_group"] not in groups:
+    primary_group = data.get("primary_group", None)
+
+    if primary_group and primary_group not in groups:
         return bad_request("Primary group does not exist")
 
     user_id = req.match_info["user_id"]
['virtool/api/users.py']