Ommit public_key argument if None

With newer Juju versions the authorized-keys
argument is expected to not be None, therefore it
now has to be ommited as an argument if thats the
case.

Change-Id: I0adeda2ce71df5505cfaaab98ec237611918d9e7
Signed-off-by: Dominik Fleischmann <dominik.fleischmann@canonical.com>