adds additional condition to PathUtils#split_subpath to avoid undefined method error for 'start_with?'

Author: ddink

lib/sprockets/path_utils.rb

@@ -162,7 +162,7 @@ def set_pipeline(path, mime_exts, pipeline_exts, pipeline)
     def split_subpath(path, subpath)
       return "" if path == subpath
       path = File.join(path, ''.freeze)
-      if subpath.start_with?(path)
+      if !subpath.nil? && subpath.start_with?(path)
         subpath[path.length..-1]
       else
         nil