check if stack output map is empty before accessing

[anjannath]

with this, we get an error instead of a panic

% ./out/mapt aws fedora create --spot --airgap --nested-virt --cpus 6 --memory 16 --project-name aws-mapt-fedora-test --backed-url file:///Users/anath/workspace --conn-details-output /tmp/fedora --tags user=anath
DEBU running 'mapt aws fedora create'
DEBU context initialized for mapt6ca81181
DEBU checking stack spotOption-aws-mapt-fedora-test
ERRO Stack outputs are empty please destroy and re-create

fixes #327