Django list_filter in Admin

Hi all,
i would like to add in my admin page a custom filter that can allow me to
select all user that doesn't belong to none groups.
If i add :

list_filter = ('groups')

it show me a filter in the right part af admin interface:

By group

All
group1
group2

i would like to add :

All
None (0)
group1
group2

is it possible?

Thanks

Francesco

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