fix in merge.rb: refine the checking if an argument is an empty string
fix in merge.rb: refine the checking if an argument is an empty string

puppetlabs/puppetlabs-stdlib,jbondpdx/puppetlabs-stdlib,yadavnikhil/puppetlabs-stdlib,DavidS/puppetlabs-stdlib,HelenCampbell/puppetlabs-stdlib,fuel-infra/puppetlabs-stdlib