[Tenable IO] Drop empty events in vulnerability data stream

[moxarth-rathod]

Proposed commit message

tenable_io: drop empty events in vulnerability data stream

The CEL program emits empty events ([{}]) on two fallback paths, 
causing the error field [original] not present as part of path [event.original]. 
Add a drop-event processor in CEL to discard empty events.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.
• I have verified that any added dashboard complies with Kibana's Dashboard good practices

Related issues

• Closes [tenable_io] Empty events in vulnerability data stream cause event.original pipeline errors #17608

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• Commit: ee694e8

cc @moxarth-rathod

[efd6]

Can we make this more explicit? Instead of using an empty sentinel, use [{"retry": true}] and then have - drop_event.when.equals.retry: true in the processors. This makes it easier to see the intent and to find the locations.