Graham Waplocus
The xtgem get variable has a default value for variables that are not set
{_$var|default value}
You can also use
<input name="var" required />
in your form so that html5 browsers will not accept a empty field
(ios and old mobile browsers may not support 'required')
You could also use js validation on the form page
for that see http://www.w3schools.com/js/js_validation.asp