ESQL: Support StubRelation with empty output in PropagateEmptyRelation

[kanoshiou]

This PR extends the PropagateEmptyRelation optimizer rule to handle StubRelation nodes with empty output, in addition to the existing LocalRelation support.

Previously, when using INLINE STATS after dropping all fields (such as ROW a = 12 | drop a | inline stats b = count(*)), the optimizer would create a StubRelation with empty output but fail to properly optimize aggregations over it. The PropagateEmptyRelation rule now checks for both LocalRelation with empty supplier and StubRelation with empty output, allowing correct optimization of aggregations in these scenarios.

Closes #136851

[luigidellaquila]

Thanks @kanoshiou

As I already commented here, this problem is a consequence of #133405

IMHO the best approach to solve the problem is to properly support this case (ie. pages with no blocks) in the aggregator function, rather than trying to change the plan.

I'm working to a different fix already, so I guess this PR can be closed (sorry, I know you spent quite some time on this, and the basic reproducer you posted on the issue was really helpful, I really appreciated it)

[luigidellaquila]

This should return 1, see comment above

[kanoshiou]

@luigidellaquila No worries at all! Thanks for taking the time to review this PR and for the kind words — I’m glad the reproducer was helpful. I’ll go ahead and close this PR.