Fix warning message condition for subsegment ending

[majanjua-amzn]

Currently, logs for lambda look like the following:

...
'_AWS_XRAY_DAEMON_PORT': '2000', 'LC_CTYPE': 'C.UTF-8', 'PYTHONPATH': '/var/runtime', '_X_AMZN_TRACE_ID': 'Root=1-66572db3-0f8e31d10b2f289d7faad872;Lineage=0fd2a748:0'})
[WARNING]	2024-05-29T13:29:34.205Z	e983bd69-7edd-4c92-a7f7-d697aef16a54	No subsegment to end.
[WARNING]	2024-05-29T13:29:34.205Z	e983bd69-7edd-4c92-a7f7-d697aef16a54	No subsegment to end.
END RequestId: e983bd69-7edd-4c92-a7f7-d697aef16a54
...

We want these No subsegment to end messages only to appear if there is no trace entity found at all. Instead, if there is a dummy segment (like in a Lambda Passthrough environment), we should not emit any message.

Description of changes:

• Update condition for error message appearance
• Change variable name to make it clear it can be any trace entity, not just a subsegment but also a segment

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.