Allow empty string for release in apt::source

This fixes a regression from commit 0a178c3.

Some repos don't have a release. This allows using such repos.

Tests added to make sure when release is set to an empty string the
source file is rendered correctly.

All other tests are passing.

manifests/source.pp

@@ -4,7 +4,7 @@
   Optional[String] $location           = undef,
   String $comment                      = $name,
   String $ensure                       = present,
-  Optional[String] $release            = undef,
+  Optional[String] $release            = 'UNDEF',
   String $repos                        = 'main',
   Optional[Variant[Hash]] $include     = {},
   Optional[Variant[String, Hash]] $key = undef,
@@ -19,7 +19,7 @@
 
   $_before = Apt::Setting["list-${title}"]
 
-  if ! $release {
+  if $release == 'UNDEF' {
     if $facts['lsbdistcodename'] {
       $_release = $facts['lsbdistcodename']
     } else {

spec/defines/source_spec.rb

@@ -376,7 +376,8 @@
     context 'release is empty string' do
       let :facts do
         {
-          :lsbdistid       => 'Debian',
+          :os => { :family => 'Debian', :name => 'Debian', :release => { :major => '7', :full => '7.0' }},
+					:lsbdistid       => 'Debian',
           :osfamily        => 'Debian',
           :puppetversion   => Puppet.version,
         }