build/MSVC: fix "C4003: not enough actual parameters for macro"

For the case of TV_DICTITEM_STRUCT, we can't just pass `1` because:
https://github.com/neovim/neovim/pull/8142#discussion_r175262436
> this variant will trigger array overrun warnings from various static analyzers.

src/nvim/api/private/helpers.c

@@ -567,7 +567,7 @@ static inline void typval_encode_dict_end(EncodedData *const edata)
     typval_encode_dict_end(edata)
 
 #define TYPVAL_ENCODE_CONV_RECURSE(val, conv_type) \
-    TYPVAL_ENCODE_CONV_NIL()
+    TYPVAL_ENCODE_CONV_NIL(val)
 
 #define TYPVAL_ENCODE_SCOPE static
 #define TYPVAL_ENCODE_NAME object