[Issue board sorting] Set position for unpositioning issues when view[ci skip]

Actually I think we could avoid doing it so because it does not look efficient.
But we can improve it later.

app/controllers/projects/boards/issues_controller.rb

@@ -8,6 +8,7 @@ module Projects
       def index
         issues = ::Boards::Issues::ListService.new(project, current_user, filter_params).execute
         issues = issues.page(params[:page]).per(params[:per] || 20)
+        make_sure_position_is_set(issues)
 
         render json: {
           issues: serialize_as_json(issues),
@@ -38,6 +39,12 @@ module Projects
 
       private
 
+      def make_sure_position_is_set(issues)
+        issues.each do |issue|
+          issue.move_to_end unless issue.relative_position
+        end
+      end
+
       def issue
         @issue ||=
           IssuesFinder.new(current_user, project_id: project.id)