[Filebeat] Dissect processor: add omit_empty option

[hostalp]

Describe the enhancement:
Please add the omit_empty option to the dissect processor that would skip setting fields without any values - similarly to how it works in the extract_array processor.

Describe a specific use case for the enhancement or feature:
At the moment, many fields without any values could be sent to the backend - that's unnecessary (and adds overhead) in most cases.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[botelastic]

Hi!
We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1.
Thank you for your contribution!

[hostalp]

Still relevant

[botelastic]

Hi!
We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1.
Thank you for your contribution!