(MODULES-3091) add test for is_string function

Author: tphoney

spec/functions/is_string_spec.rb

@@ -18,6 +18,7 @@
   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([]).and_return(false) }
   it { is_expected.to run.with_params([1]).and_return(false) }
   it { is_expected.to run.with_params({}).and_return(false) }