Fix Auto DevOps check domain is blank

What does this MR do?

It appears the old logic was setting the KUBE_INGRESS_DOMAIN to be an empty string when it was previously unset and as such the next check to see if it was set was resulting in us assuming it had been set.

# BEFORE

$ unset KUBE_INGRESS_BASE_DOMAIN

$ unset AUTO_DEVOPS_DOMAIN

$ if [ -z ${KUBE_INGRESS_BASE_DOMAIN+x} ]; then
>   export KUBE_INGRESS_BASE_DOMAIN=$AUTO_DEVOPS_DOMAIN
> fi

$ echo ${KUBE_INGRESS_BASE_DOMAIN+x}
x

# AFTER

$ unset KUBE_INGRESS_BASE_DOMAIN

$ unset AUTO_DEVOPS_DOMAIN

$ if [ -z ${KUBE_INGRESS_BASE_DOMAIN+x} ] && [ -n "$AUTO_DEVOPS_DOMAIN" ] ; then
>   export KUBE_INGRESS_BASE_DOMAIN=$AUTO_DEVOPS_DOMAIN
> fi

$ echo ${KUBE_INGRESS_BASE_DOMAIN+x}

This is another good case for https://gitlab.com/gitlab-org/gitlab-ce/issues/55165 so we can actually unit test this stuff.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• [-] Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #57798 (closed)