Attribute::delete_seq: Set data_size=0

As the sequence has been be freed at set to nullptr, we should also indicate that the sequence is empty by setting the data_size to zero.

This gets us back to the same state as before we called set_value.

This avoids a segfault if we call check_alarm after push_event as in catch2_alarm.cpp.

Fixes pytango#615 (closed)