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, 2 insertions, 1 deletions

diff --git a/spec/aliases/string_spec.rb b/spec/aliases/string_spec.rb
index 8e01d55..853b5af 100644
--- a/spec/aliases/string_spec.rb
+++ b/spec/aliases/string_spec.rb
@@ -6,6 +6,7 @@ if Puppet.version.to_f >= 4.0
       [
         '',
         'one',
+        nil,
       ].each do |value|
         describe value.inspect do
           let(:params) {{ value: value }}
@@ -23,7 +24,7 @@ if Puppet.version.to_f >= 4.0
       ].each do |value|
         describe value.inspect do
           let(:params) {{ value: value }}
-          it { is_expected.to compile.and_raise_error(/parameter 'value' expects a Stdlib::Compat::String = String/) }
+          it { is_expected.to compile.and_raise_error(/parameter 'value' expects a String/) }
         end
       end
     end