fix(core): Drop empty batches before they reach downstream buffers

[graphcareful]

Summary

This change drops empty batches in the fanout before they reach downstream buffers. This error is classified as a programmer error, and it can only happen if there is a bug in a transform (or source) which emits an empty batch into the pipeline.

However with disk buffering enabled if this bug does arise Vector crashes. This change drop the batch and logs, when in debug mode an assertion is instead raised.

How did you test this PR?

Additional unit tests

Change Type

• Bug fix
• New feature
• Dependencies
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

Notes

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[bruceg]

Makes sense to me.