Fix event creation without physical location

[MizukiTemma]

Short description

This PR fixes the problem that events cannot be created without POI even if "Event does not have a physical location" is checked.

Proposed changes

• Clean the POI id before passing the user input values into the form.: the cause was "-1", which is passed as POI id if no POI is selected but causes validation error, resulting in the form is judged as invalid (before reaching clean method).
• Add a test for this case

Side effects

• I hope none 🙈

Resolved issues

Fixes: #3223

---

Pull Request Review Guidelines

[lunars97]

Everything looks good and works as expected! Thank you for fixing it 💛

[david-venhoff]

Thanks, also for the tests!

Looks like the bug is that an element can not be found anymore. I get this console error when I click the "has not location" checkbox:

The commit that introduced this bug is 6fb7122

There, the id field for the input was probably accidentally removed, which means that the js code that controls it cannot find it anymore.

The fix is to add this to _poi_address_container.html line 7:

id="id_location

Then we don't need this extra code on the python side :)

(For reference, what the js code does is that it adds the disabled attribute to the location input, which means that it will not be part of the form data when the form is submitted, and that means that the python code does not receive the -1 value at all)