Add default value in InputGroupTrait.php
Add default value in InputGroupTrait.php

Adds a default null value for templateVars in InputGroupTrait so 
https://github.com/FriendsOfCake/bootstrap-ui/blob/6924ce44d72af0346998054f715907682210ffd7/src/View/Widget/InputGroupTrait.php#L83 does not print a notice. 

Fixes https://github.com/FriendsOfCake/bootstrap-ui/issues/370
FriendsOfCake/bootstrap-ui,FriendsOfCake/bootstrap-ui