skip mentions if text is nil (can happen during bulk updates) (refs #8)

lib/redmine_mattermost/listener.rb

@@ -256,6 +256,7 @@ private
 	end
 
 	def mentions text
+		return nil if text.nil?
 		names = extract_usernames text
 		names.present? ? "\nTo: " + names.join(', ') : nil
 	end