'@' is not a bit. There are two completely separate uses of OBJECT_NULL_BIT.
When m_data[13] gets set to OBJECT_NULL_BIT, you can set it to '@' or whatever byte you like.
When storage[0] of inline values is set to OBJECT_NULL_BIT and then tested by anding with OBJECT_NULL_BIT you are going to get false positive nulls if it's more than just a bit. Actually, I'd like to get rid of this use of logical anding and reserve the single special value 0xFF for the INLINE_OBJECT_NULL_BYTE. That will make it easier in the future to inline up to 254 bytes (63 characters instead of just 63 bytes).