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
 

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

src/nvim/eval/encode.c

@@ -287,6 +287,9 @@ int encode_read_from_list(ListReaderState *const state, char *const buf,
       (val)->copyID_attr = copyID; \
     } while (0)
 
+#define TV_STRLEN(tv) \
+    (tv->vval.v_string == NULL ? 0 : STRLEN(tv->vval.v_string))
+
 /// Define functions which convert VimL value to something else
 ///
 /// Creates function `vim_to_{name}(firstargtype firstargname, typval_T *const
@@ -306,7 +309,7 @@ static int name##_convert_one_value(firstargtype firstargname, \
 { \
   switch (tv->v_type) { \
     case VAR_STRING: { \
-      CONV_STRING(tv->vval.v_string, STRLEN(tv->vval.v_string)); \
+      CONV_STRING(tv->vval.v_string, TV_STRLEN(tv)); \
       break; \
     } \
     case VAR_NUMBER: { \