Fix the check for whether GitLab-Exporter is enabled

Summary

Fixes the check for GitLab Exporter being enabled (follow-up to !238 (merged)).

This caused a bug where properly setting gitlab.gitlab-exporter.enabled=false meant that Exporter resources were not included in the Helm template, but the Operator still returned GitLabExporterEnabled=true (because that's the default), and the controller would crash with a nil pointer exception.

Closes #523 (closed)