Updated conf.erb
Updated conf.erb

Avoid to set TARGET_USER and TARGET_PASS to empty strings if they are not set.
Duply check if these vaules are set and in that case it will add "@TARGET_USER:TARGET_PASS" to the TARGET url, causing an error if they are empty strings
tohuwabohu/puppet-duplicity,Samgarr/puppet-duplicity,tohuwabohu/puppet-duplicity,Samgarr/puppet-duplicity