Protect against nil due_on value in imported data

lib/gitlab/github_import/importer/milestones_importer.rb

@@ -42,7 +42,7 @@ module Gitlab
             description: milestone.description,
             project_id: project.id,
             state: state_for(milestone),
-            due_date: milestone.due_on.to_date,
+            due_date: milestone.due_on&.to_date,
             created_at: milestone.created_at,
             updated_at: milestone.updated_at
           }