Fix TypeError in CountryFilter.choices()

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")

django_countries/filters.py

@@ -24,7 +24,7 @@ def choices(self, changelist):
         }
         for lookup, title in self.lookup_choices(changelist):
             if django.VERSION >= (5, 0):
-                selected = force_str(lookup) in value
+                selected = value is not None and force_str(lookup) in value
             else:
                 selected = force_str(lookup) == value
             yield {