Fix instance project templates no longer working

Merged Stan Hu requested to merge sh-fix-instance-templates-not-working into master

The introduction of group project templates in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7391 caused instance project templates to stop working. The frontend submits an empty group_with_project_templates_id parameter, which is interpreted as an actual value to search. We fix this by ignoring blank parameters.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/9038