default for outbound_local_requests_whitelist is nil

Summary

outbound_local_requests_whitelist does not have a default value when GDK is already installed

What is the current bug behavior?

This probably will happen in other places, but it happened to me when I tried to view Prometheus metrics on a project. I clicked on Metrics and got undefined method each for nil:NilClass

What is the expected correct behavior?

outbound_local_requests_whitelist should default to [] to prevent the nil error.

Possible fixes

I believe the fix may be in adding the default to this migration (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30350/diffs#60711f772ca8660ca94f5bdd93615683f26e0244_0_7)