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.h

@@ -122,6 +122,8 @@ typedef enum {
     VV_FALSE,
     VV_TRUE,
     VV_NULL,
+    VV__NULL_LIST,  // List with NULL value. For test purposes only.
+    VV__NULL_DICT,  // Dictionary with NULL value. For test purposes only.
 } VimVarIndex;
 
 /// All recognized msgpack types