Return nil in Document#block_delimiter rather than an empty array.

The block delimiter is normally a string, so nil makes more sense when
there is no delimiter. We also don't want to allocate an array for no
reason.

lib/liquid/document.rb

@@ -8,7 +8,7 @@ module Liquid
 
     # There isn't a real delimiter
     def block_delimiter
-      []
+      nil
     end
 
     # Document blocks don't need to be terminated since they are not actually opened