Fix projected import failing on missing relations

lib/gitlab/import_export/project_tree_restorer.rb

@@ -52,7 +52,11 @@ module Gitlab
           create_sub_relations(relation, @tree_hash) if relation.is_a?(Hash)
 
           relation_key = relation.is_a?(Hash) ? relation.keys.first : relation
-          relation_hash = create_relation(relation_key, @tree_hash[relation_key.to_s])
+          relation_hash_list = @tree_hash[relation_key.to_s]
+
+          next unless relation_hash_list
+
+          relation_hash = create_relation(relation_key, relation_hash_list)
           saved << restored_project.append_or_update_attribute(relation_key, relation_hash)
         end
         saved.all?