puppet only exec when file does NOT exist

I only want to exec following command when file (/usr/local/bin/papply) does not exist. not sure what to put there. exec { 'git add url': command =>'git remote add origin https://github.com/testing/puppet.git', require => Exec['git init'], cwd => '/home/vagrant/django', user => 'vagrant', onlyif => "not sure what to put here" }