Allow OpensearchTaskHandler and OpensearchRemoteLogIO to take empty username and password

[topherinternational]

This change allows OpensearchTaskHandler and OpensearchRemoteLogIO to utilize Opensearch connections without user/password authentication.

Current Behavior
These classes pass their username and password attributes to the OpenSearch client constructor. In the event these attributes are empty or null, the client attempts to connect to the host using an auth string of ":" or "None:None".

Proposed Behavior
os_task_handler._create_opensearch_client() passes the http_auth tuple to the OpenSearch() client constructor only if both username and password are truthy (i.e., they are non-empty strings).

Rationale
Several use cases take advantage of a non-authenticated endpoint. Opensearch instances are sometimes deployed without authentication for testing purposes, and production instances can be operated inside of a VPC where networking rules rather than credentials control access from other services in the cluster.

Additionally, this non-authenticated capability is already in place in the OpenSearchHook.

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