Fix instance project templates no longer working

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

ee/app/services/ee/projects/create_from_template_service.rb

@@ -28,10 +28,14 @@ module EE
 
       def template_project
         strong_memoize(:template_project) do
-          current_user.available_custom_project_templates(search: template_name, subgroup_id: params[:group_with_project_templates_id])
+          current_user.available_custom_project_templates(search: template_name, subgroup_id: subgroup_id)
                       .first
         end
       end
+
+      def subgroup_id
+        params[:group_with_project_templates_id].presence
+      end
     end
   end
 end

ee/changelogs/unreleased/sh-fix-instance-templates-not-working.yml

+---
+title: Fix instance project templates no longer working
+merge_request: 9019
+author:
+type: fixed

ee/spec/services/projects/create_from_template_service_spec.rb

@@ -69,9 +69,10 @@ describe Projects::CreateFromTemplateService do
       end
     end
 
-    context 'creates project from custom template' do
+    shared_examples 'creates project from custom template' do |subgroup_id|
       # If we move the project inside a let block it throws a SEGFAULT error
       before do
+        project_params[:group_with_project_templates_id] = subgroup_id
         @project = subject.execute
       end
 
@@ -91,6 +92,9 @@ describe Projects::CreateFromTemplateService do
       end
     end
 
+    it_behaves_like 'creates project from custom template', nil
+    it_behaves_like 'creates project from custom template', ''
+
     describe 'creating project from a Group project template', :postgresql do
       let(:project_name) { project_template.name }
       let(:group_with_project_templates_id) { subgroup_1.id }