yesno filter: just don't return None ever

I had someone run into this on hosted Bugsink; couldn't reproduce it.
Thought I fixed it in e8fb9556f7 (specific to the Chinese translation)
but appararently there's other ways to reach this point.

No matter, just create a version of the yesno filter that's not sensitive
to any future mistranslation.

projects/forms.py

@@ -1,12 +1,12 @@
 from django import forms
 from django.contrib.auth import get_user_model
-from django.template.defaultfilters import yesno
 from django.urls import reverse
 from django.utils.translation import gettext_lazy as _
 from django.utils.html import format_html
 
 from bugsink.utils import assert_
 from teams.models import TeamMembership
+from bsmain.utils import yesno
 
 from .models import Project, ProjectMembership, ProjectRole