Unable to clear session token from Azure DevOps AWS Service Connection

**Describe the bug**

Unable to clear out session token from Azure DevOps's service connection, if it was previously assigned.
I have to delete and recreate the service connection to get around this issue.

**To reproduce**

1. Create a new service connection, and enter a valid access key id & secret access key
2. Run pipeline with AWS task, which will run successfully
3. Edit the service connection, enter an invalid session token
4. Rerun same pipeline, which will fail as expected
5. Edit the service connection, reenter the valid access key id & secret access key, and clear the session token field.
6. Rerun pipeline, failed with the error message: An error occurred (UnrecognizedClientException) when calling the GetAuthorizationToken operation: The security token included in the request is invalid

**Expected behavior**

The 3rd attempt to run should success, with only the access key id & secret key passed on to aws task

**Screenshots**


**Your Environment**

- On-prem or cloud based? Cloud
- Azure DevOps version:
- AWS Toolkit for Azure DevOps version: 1.13.0

**Additional context**

- Tested using an IAM account's credential without key rotation

Start by reproducing the documented service-connection sequence and trace how the AWS task reads the access key, secret key, and session token after the token field is cleared. Done means rerunning the pipeline succeeds with only the access key id and secret access key passed to the AWS task.

Written by the indexing model from the issue text.