Fix TypeError in CountryFilter.choices()

[foutrelis]

When no country is selected, self.used_parameters.get(self.field.name) will return None. This raises a TypeError when CountryFilter.choices() tries to check whether a specific country choice is currently selected.

Avoid this by checking that the parameter is not empty similarly to how Django's built-in admin.filters.RelatedFieldListFilter checks for this.

Fixes: 04662d1 ("Support Django 5.0")

[foutrelis]

The issue is reproducible with something simple like:

from django.db import models
from django_countries.fields import CountryField

class Person(models.Model):
    name = models.CharField(max_length=100)
    country = CountryField()

from django.contrib import admin
from django_countries.filters import CountryFilter

from .models import Person

@admin.register(Person)
class PersonAdmin(admin.ModelAdmin):
    list_filter = [("country", CountryFilter)]

[claudep]

I guess a regression test is required here.

[foutrelis]

I guess a regression test is required here.

Good suggestion, thanks. I extended the test for this method to also check without a selected country.

[SmileyChris]

Thanks! I merged and released a new point version with your fix.