If reference_pattern is nil, set to empty string

jh/app/models/integrations/ones.rb

@@ -62,6 +62,10 @@ def fields
       OnesFields.fields
     end
 
+    def self.reference_pattern(only_long: true)
+      @reference_pattern ||= ""
+    end
+
     def issue_tracker_path
       url
     end