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 { \

src/nvim/lua/executor.c

@@ -944,6 +944,7 @@ static void nlua_print_event(void **argv)
       break;
     }
     msg(str + start);
+    msg_didout = true;  // Make blank lines work properly
   }
   if (len && str[len - 1] == NUL) {  // Last was newline
     msg("");