Permit undef passed as nil to validate_string

[domcleal]

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'

(Introduced by #659)

[DavidS]

Thanks!