calculate undef dependant on version

Author: tphoney

spec/functions/is_string_spec.rb

@@ -1,6 +1,10 @@
 require 'spec_helper'
 
 describe 'is_string' do
+  let(:undef_value) do
+    (Puppet::Util::Package.versioncmp(Puppet.version, '6.0.0') < 0) ? :undef : nil
+  end
+
   it { is_expected.not_to eq(nil) }
   it { is_expected.to run.with_params.and_raise_error(Puppet::ParseError, %r{wrong number of arguments}i) }
   it {
@@ -18,7 +22,7 @@
   it { is_expected.to run.with_params(-3.7).and_return(false) }
   it { is_expected.to run.with_params('-3.7').and_return(false) }
 
-  it { is_expected.to run.with_params(undef).and_return(false) }
+  it { is_expected.to run.with_params(undef_value).and_return(false) }
   it { is_expected.to run.with_params([]).and_return(false) }
   it { is_expected.to run.with_params([1]).and_return(false) }
   it { is_expected.to run.with_params({}).and_return(false) }