eval/encode: Make sure that encoder can encode NULL variables

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.

src/nvim/eval.c

@@ -379,6 +379,8 @@ static struct vimvar {
   VV(VV_FALSE,          "false",            VAR_SPECIAL, VV_RO),
   VV(VV_TRUE,           "true",             VAR_SPECIAL, VV_RO),
   VV(VV_NULL,           "null",             VAR_SPECIAL, VV_RO),
+  VV(VV__NULL_LIST,     "_null_list",       VAR_LIST, VV_RO),
+  VV(VV__NULL_DICT,     "_null_dict",       VAR_DICT, VV_RO),
 };
 #undef VV