hbacrule: Allow clearing members with empty lists.

If a hbacrule member has any value, the only way to clear it is by
creating a task with 'state: absent' and 'action: member' and provide
a list with all the values for that member.

This patch allows the use of '<member>: []' with 'action: hbacrule'
to clear a hbacrule member.

A new test playbook can be found at:

    tests/hbacrule/test_hbacrule_member_empty.yml