juju scp and ssh fail with arg --proxy when no models are in focus

[Christopher Lee]

When using either scp or ssh subcommand with the '--proxy' argument when no model is in focus causes the command to error and fail.

To reproduce:

$ juju bootstrap lxd/localhost proxybug-test
$ juju add-model testing-proxy
$ juju scp -m proxybug-test:controller 0:/var/log/syslog /tmp/ # This works
$ juju scp --proxy -m proxybug-test:controller 0:/var/log/syslog /tmp/ # This works too

$ juju destroy-model -y testing-proxy
# Now no models are in focus.

$ juju scp -m proxybug-test:controller 0:/var/log/syslog /tmp/ # This works
$ juju scp --proxy -m proxybug-test:controller 0:/var/log/syslog /tmp/ # This DOES NOT work
ERROR No model in focus.

[Menno Finlay-Smits (menno.smits)]

Thanks for the excellent bug report. I can repro this (with `juju ssh` too).

[Menno Finlay-Smits (menno.smits)]

Fix: https://github.com/juju/juju/pull/6977