Permit undef passed as `nil` to validate_string

When validate_string is called via the Puppet 4 deprecation wrappers
from deprecation_gen (introduced in 970852d), `undef` is passed as `nil`
where it was previously passed as `''` from the Puppet 3-style function
API.

This change explicitly permits a `nil` value in validate_string, and
adds a test case to `is_string` which also accepts the same.

Fixes test failures in apt, concat etc:

    Error while evaluating a Function Call, nil is not a string.  It looks to be a NilClass at apt/manifests/source.pp:23:3 [..]
    # ./spec/fixtures/modules/stdlib/lib/puppet/parser/functions/validate_string.rb:34:in `block (2 levels) in <module:Functions>'
    # ./spec/fixtures/modules/stdlib/lib/puppet/parser/functions/validate_string.rb:32:in `each'
    # ./spec/fixtures/modules/stdlib/lib/puppet/parser/functions/validate_string.rb:32:in `block in <module:Functions>'
    # puppet-4.7.0/lib/puppet/parser/functions.rb:174:in `block (2 levels) in newfunction'
    # puppet-4.7.0/lib/puppet/util/profiler/around_profiler.rb:58:in `profile'
    # puppet-4.7.0/lib/puppet/util/profiler.rb:51:in `profile'
    # puppet-4.7.0/lib/puppet/parser/functions.rb:167:in `block in newfunction'
    # ./spec/fixtures/modules/stdlib/lib/puppet_x/puppetlabs/stdlib/deprecation_gen.rb:13:in `block (2 levels) in deprecation_gen'

1 files changed, 11 insertions, 0 deletions

diff --git a/spec/acceptance/is_string_spec.rb b/spec/acceptance/is_string_spec.rb
index 94d8e96..f526888 100755
--- a/spec/acceptance/is_string_spec.rb
+++ b/spec/acceptance/is_string_spec.rb
@@ -95,6 +95,17 @@ describe 'is_string function', :unless => UNSUPPORTED_PLATFORMS.include?(fact('o
         expect(r.stdout).to match(/Notice: output correct/)
       end
     end
+    it 'is_strings undef' do
+      pp = <<-EOS
+      $a = undef
+      $o = is_string($a)
+      notice(inline_template('is_string is <%= @o.inspect %>'))
+      EOS
+
+      apply_manifest(pp, :catch_failures => true) do |r|
+        expect(r.stdout).to match(/is_string is true/)
+      end
+    end
   end
   describe 'failure' do
     it 'handles improper argument counts'