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.h

@@ -158,6 +158,7 @@ typedef enum {
     // Neovim
     VV_STDERR,
     VV_MSGPACK_TYPES,
+    VV__NULL_STRING,  // String with NULL value. For test purposes only.
     VV__NULL_LIST,  // List with NULL value. For test purposes only.
     VV__NULL_DICT,  // Dictionary with NULL value. For test purposes only.
     VV_LUA,