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 {