Update tests.

manifests/config.pp

@@ -34,6 +34,8 @@ define postfix::config ($value = undef, $ensure = 'present') {
     "\$ensure must be either 'present', 'absent' or 'blank', got '${ensure}'")
   if ($ensure == 'present') {
     validate_string($value)
+    validate_re($value, '^.+$',
+      '$value can not be empty if ensure = present')
   }
 
   if (!defined(Class['postfix'])) {

spec/defines/postfix_config_spec.rb

@@ -11,7 +11,7 @@ describe 'postfix::config' do
     it 'should fail' do
       expect {
         should contain_augeas("set postfix 'foo'")
-      }.to raise_error(Puppet::Error, /Must pass value to Postfix::Config/)
+      }.to raise_error(Puppet::Error, /value can not be empty/)
     end
   end
 
@@ -46,7 +46,7 @@ describe 'postfix::config' do
     it 'should fail' do
       expect {
         should contain_augeas("set postfix 'foo'")
-      }.to raise_error(Puppet::Error, /must be either 'present' or 'absent'/)
+      }.to raise_error(Puppet::Error, /must be either 'present', 'absent' or 'blank'/)
     end
   end