fix(fiat): Fix access denied for new apps with no roles

[costimuraru]

Fixes spinnaker/spinnaker#1930

Previous behavior (with LDAP enabled):

1. User creates Spinnaker app and specifies no roles.
2. Front50 does not make call to Fiat to sync permissions (this is because the roles list is empty).
3. User tries to view app.
4. Front50 makes call to Fiat to get permissions for the new app.
5. Fiat doesn't have any entry for the app (in Redis) and as such AccessDeniedException: Access is denied is thrown.
6. After 10 minutes Fiat will sync permissions for all apps (it has a service to sync these periodically) and the Access denied error will be resolved.

Behavior after this PR:

1. User creates Spinnaker app and specifies no roles.
2. Front50 makes call to Fiat to sync permissions (even though permission list is empty).
3. Fiat generates entry for the new app (in Redis).
4. User tries to view app.
5. Front50 makes call to Fiat to get permissions for the new app.
6. Fiat is able to read the entry for the new app (from Redis) and the Access denied is no longer thrown.

[costimuraru]

ping @robfletcher @ttomsu @ajordens @robzienert @asher

Relates to 8250c54#diff-3d176880bdd6eb3033f247b7792c7508

[robzienert]

Definitely the type of change that should have a test along with it. If / when we get to refactoring this away from Groovy, we would be likely introduce a regression with this conditional due to incorrectly translating Groovy Truth. :(

ed: Change itself seems legit, though.

[costimuraru]

Good point, @robzienert!
I've added the tests now.