eval: add v:_null_string

Replacement for Vim's test_null_string().
Vim uses it to verify that its codebase handles null strings.
Preparation for the Test_null_list() in patch v8.2.1822.

Use v:_null_string, not non-existent env var, for null string tests.

Mention v:_null_string in id() because id(v:_null_string) returns (nil).

src/nvim/eval.c

@@ -229,6 +229,7 @@ static struct vimvar {
   // Neovim
   VV(VV_STDERR,         "stderr",           VAR_NUMBER, VV_RO),
   VV(VV_MSGPACK_TYPES,  "msgpack_types",    VAR_DICT, VV_RO),
+  VV(VV__NULL_STRING,   "_null_string",     VAR_STRING, VV_RO),
   VV(VV__NULL_LIST,     "_null_list",       VAR_LIST, VV_RO),
   VV(VV__NULL_DICT,     "_null_dict",       VAR_DICT, VV_RO),
   VV(VV_LUA,            "lua",              VAR_PARTIAL, VV_RO),