py/builtinhelp: Don't print removed sentinel entries.

This fixes the test used by the help function to iterate over its
argument's attribute to use the proper `mp_map_slot_is_filled` function
to check if a slot in the map is filled; the previous test only checked
for `MP_OBJ_NULL` keys and would attempt to print the null value
whenever a `MP_OBJ_SENTINEL` key marking a deleted entry was present.

Fixes: micropython#18061
Fixes: micropython#18481

Signed-off-by: Anson Mansfield <amansfield@mantaro.com>

Author: AJMansfield

py/builtinhelp.c

@@ -152,9 +152,8 @@ static void mp_help_print_obj(const mp_obj_t obj) {
     }
     if (map != NULL) {
         for (uint i = 0; i < map->alloc; i++) {
-            mp_obj_t key = map->table[i].key;
-            if (key != MP_OBJ_NULL) {
-                mp_help_print_info_about_object(key, map->table[i].value);
+            if (mp_map_slot_is_filled(map, i)) {
+                mp_help_print_info_about_object(map->table[i].key, map->table[i].value);
             }
         }
     }