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