`CustomListValue::CustomListValue()` set's it into nullptr and it triggers
`-fsanitize=nullability`.

There are not deference of this pointer, it used only in comparison. Marking it
`nonnull` does not help to protect dereferences.

Also mark `arena` as const, to make sure it's not used for allocation, those
methods are non-const.

PiperOrigin-RevId: 802145149