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.

spec/defines/source_spec.rb

@@ -373,6 +373,19 @@
       end
     end
 
+    context 'release is empty string' do
+      let :facts do
+        {
+          :lsbdistid       => 'Debian',
+          :osfamily        => 'Debian',
+          :puppetversion   => Puppet.version,
+        }
+      end
+      let(:params) { { :location => 'hello.there', :release => '' } }
+
+      it { is_expected.to contain_apt__setting('list-my_source').with_content(/hello\.there  main/) }
+    end
+
     context 'invalid pin' do
       let :facts do
         {