[fix] fix some AATH flow where a connection does not yet exist

[gmulhearn]

continues from changes in #1278, with a small change to not use the "get connection by sender" API, since the connection may not exist in some contexts.

This should fix the recent AATH issue being reported by @nodlesh :

riesVCX Error: Error: Error: POST failed due to non-success HTTP status: 500 Internal Server Error, response body: Error: AgentError: Invalid state: Found no connections by sender's verkey 4rBrKy5znEKSvePtLjsv8ws2ERteF6vjcdabb1PxtrkL
  
�[90m[�[0m2024-08-05T03:14:02Z �[1m�[31mERROR�[0m aries_vcx_backchannel::error�[90m]�[0m Error: AgentError: AriesVCX error: AriesVCX Error: Error: Error: POST failed due to non-success HTTP status: 500 Internal Server Error, response body: Error: AgentError: Invalid state: Found no connections by sender's verkey 4rBrKy5znEKSvePtLjsv8ws2ERteF6vjcdabb1PxtrkL

[nodlesh]

Looks good. I didn't run the tests, so we will see in the nightly runs.

[JamesKEbert]

This looks good from a quick review, but I did not run the tests