You could try this:
{% if not profile.user.first_name.value %}
<p> -- </p>
{% else %}
{{ profile.user.first_name }} {{ profile.user.last_name }}
{% endif %}
This way, you're essentially checking to see if the form field first_name
has any value associated with it. See {{ field.value }}
in Looping over the form's fields in Django Documentation.
I'm using Django 3.0.