Adds two undocumented v: variables: _null_list and _null_dict because I do not
know a reproducible way to get such lists (though I think I heard about this)
and dictionaries (do not remember hearing about them). NULL strings are obtained
using $XXX_UNEXISTENT_VAR_XXX.
Fixes crash in json_encode($XXX_UNEXISTENT_VAR_XXX). Other added tests worked
fine before this commit.
