Fix box-sizing deprecation warning 
Fix box-sizing deprecation warning 

Do not try to output any box-sizing if no $model (null, false) is passed in.
rajkaushik05/susy,Sapphire64/susy,gabskoro/susy,pairyo/susy,oddbird/susy,rajkaushik05/susy,rainforestapp/susy,ericam/susy,kaij/susy,BionicClick/susy,kaij/susy,kraftner/susy,greyhwndz/susy,Sapphire64/susy,oddbird/susy,ericam/susy,greyhwndz/susy,pairyo/susy,Sapphire64/susy,BionicClick/susy,rajkaushik05/susy,kraftner/susy,ericam/susy,rainforestapp/susy,rainforestapp/susy,pairyo/susy,kaij/susy,greyhwndz/susy,gabskoro/susy,kraftner/susy,katoffsky/susy,BionicClick/susy,gabskoro/susy,katoffsky/susy,katoffsky/susy