Fix unbound variable
Fix unbound variable

Now that we have implemented the slim dev deployment functionality, we
should make sure it doesn't break any of our persisting environments.

Falling back to nothing with bash should ensure the desired behaviour,
where it has failed on us in previous staging deployment.

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf