Empty string for "warn_on" causes failure

[jakemac53]

If you do warn_on: "", then the job fails with FormatException: "--ignore_license" is not an allowed value for option "warn_on"..

Presumably, it ends up passing --warn_on --ignore_license with nothing between since the string is empty. The argument shouldn't be passed at all or something probably.

[jakemac53]

Fwiw, in general it is difficult to just move a job from default warning to default failure. You have to override the entire setting, with all the default failure jobs, and also override the warn_on settings to remove the job from that, but leave the old ones.