For posterity, I have a few NullBooleanFields and here’s what I do:
To check if it’s True:
{% if variable %}True{% endif %}
To check if it’s False (note this works because there’s only 3 values — True/False/None):
{% if variable != None %}False{% endif %}
To check if it’s None:
{% if variable == None %}None{% endif %}
I’m not sure why, but I can’t do variable == False, but I can do variable == None.