fix(api, lua): make blank lines in a message work properly

(cherry picked from commit e6bb22e691)

src/nvim/api/vim.c

@@ -1686,6 +1686,7 @@ static void write_msg(String message, bool to_err, bool writeln)
   if (c == NL) { \
     kv_push(line_buf, NUL); \
     msg(line_buf.items); \
+    msg_didout = true; \
     kv_drop(line_buf, kv_size(line_buf)); \
     kv_resize(line_buf, LINE_BUFFER_MIN_SIZE); \
   } else { \