[to_bitmap] the behavior of to_bitmap() function is inconsistent

[morningman]

Describe the bug

case 1:

mysql> select to_bitmap(null);
+-----------------+
| to_bitmap(NULL) |
+-----------------+
| NULL            |
+-----------------+
1 row in set (0.01 sec)

It returns NULL because for null parameter, the function will return null. (directly rewrote by FE planner).

Case 2:

mysql> select bitmap_union_count(to_bitmap(k2)) from tbl5;
+-------------------------------------+
| bitmap_union_count(to_bitmap(`k2`)) |
+-------------------------------------+
|                                   0 |
+-------------------------------------+
1 row in set (0.02 sec)

The column k2 in tbl5 is null. And to_bitmap(k2) return an empty bitmap.

Case 3:

mysql> select bitmap_union_count(to_bimap(null)) from tbl5;
ERROR 1064 (HY000): errCode = 2, detailMessage = No matching function with signature: to_bimap(unknown type: NULL_TYPE).

I expected this return 0, but error returns.

Expected behavior

Should the method to_bitmap() be null protected? Which means to_bitmap(null) should always return an empty bitmap, not null.