Skip non-existing paths in $MODULEPATH silently

In `map`, a `next` will leave a `nil` value in the array.  This is a
problem since `FileTest.directory? nil` raises "TypeError: no implicit
conversion of nil into String"

Bug was introduced in v4.0.1, in commit b6bfd6f

lib/puppetlabs_spec_helper/module_spec_helper.rb

@@ -58,7 +58,7 @@ def verify_contents(subject, title, expected_lines)
 
   Dir.entries(dir).grep_v(/^\./).map { |f| File.join(dir, f, 'spec', 'lib') }
 end
-components.flatten.each do |d|
+components.compact.flatten.each do |d|
   $LOAD_PATH << d if FileTest.directory?(d) && !$LOAD_PATH.include?(d)
 end