Fix AWS CloudWatch sequence token handling for new/empty log streams

[errordeveloper]

Description: Handle nil sequence token for empty log streams
Link to tracking Issue: #4683

[errordeveloper]

/cc @mxiamxia @Aneurysm9

[mxiamxia]

In most of CW Logs use case, the log group/stream needs to be created in flight for the first time use. I think we should create Stream based on Config instead of failing the exporter.

[mxiamxia]

CWLogs exporter is in pretty early stage. There are a few MUST to be done things,

1. Batch log events and make sure the batch size is less then 1MB when calls PutLogsEvent API
2. each single logs event size should be less than 256KB
3. Create Log Group/Stream in flight from Config if it is not yet found in CW logs backend.
4. Gracefully handle CW Logs API exceptions in response
5. ...

[mxiamxia]

Created an issue to track TODO for CWLogs Exporter. I am fine to merge this PR.