Messages with an empty ~thread decorator

[usingtechnology]

0.8.1 is sending messages with an empty ~thread.

This can confuse callers as RFC 0008 appears ambiguous; it states what to do if ~thread has not thid and what to do if no ~thread, but not what to do with ~thread with no thid.

If ACA-Py has no data in the ~thread, then let's not include it.

See hyperledger/aries-vcx#875

[usingtechnology]

After looking through ACA-Py code, it is pretty clear that thid is not always expected to be populated in a ~thread, so I think we need to do this on a case-by-case basis.

However, the code clearly shows that thid was intended to be populated for credential-offer and there is a bug here. I will file a PR to address that bug.