Filter "Niet bepaald" at Stadsdelen does not work properly

[someren]

Describe the bug
You can filter on Stadsdelen or simply choose the option "Niet bepaald". The result shows signals/tickets which do have a Stadsdeel. This filter does not work correctly. I expect a list with signals/tickets which do not have a Stadsdeel.

To Reproduce
Steps to reproduce the behavior:

1. Go to Filteren
2. Click on Stadsdeel > Niet bepaald
3. Click on "Filteren"
4. See list with tickets/signals which have a Stadsdeel instead of an empty one.

Expected behavior
I expect to create a list with tickets/signals that do not have a Stadsdeel and are somewhat 'lost'.

Screenshots
Filter:

Result (is from acceptatieomgeving, production showed only 1 result):

Desktop (please complete the following information):

• OS: Microsoft
• Browser: Chrome

[CBuiVNG]

this is a back-end issue. I've made a PR Amsterdam/signals#756 to correct this

[tcoenen]

I saw this still does not work correctly, there are a few issues. One is that the backend does not accept area_type_code=null to filter on, for which I have a PR open: Amsterdam/signals#932

The other issue is that the frontend erroneously filters on area_type_code=district for Signal instances that have no area determined. If "Signalen" was not able to determine in which area a Signal (nuisance complaint) lies it will not set either of the area_code and area_type_code properties for that Signal's location. The aforementioned filtering will not work because area_type_code will have value null (and not district) in the database.

[someren]

Getest op versie 2.4.1, issue speelt nog steeds