Pass empty string instead of nil for starting directory entry.

Author: aardvark179

src/main/ruby/truffleruby/core/dir_glob.rb

@@ -181,7 +181,7 @@ def process_directory(matches, parent, entry, glob_base_dir)
           switched = @next.dup
           switched.separator = @separator
         else
-          @next.process_entry entry, true, matches, parent, glob_base_dir if glob_base_dir
+          @next.process_entry '', true, matches, parent, glob_base_dir if glob_base_dir
         end
 
         stack = [nil]