Correctly handle partition transforms without output

[lava]

The case where a partition transform removed all input data
and returned an empty output (which is important when deleting
partitions) was not handled correctly, leading to potential
segfaults.

This was not caught by test because we lacked an end-to-end test for whole-partition erasure, with existing tests all mocking the partition transformer.

📝 Checklist

• All user-facing changes have changelog entries.
• The changes are reflected on docs.tenzir.com/vast, if necessary.
• The PR description contains instructions for the reviewer, if necessary.

🎯 Review Instructions

[tobim]

I only reviewed the unit test so far.

[tobim]

I think the change fixes the issue but the code needs a bit of cleanup before it can be merged.