[SPARK-56472][SQL] Fix MERGE schema evolution with WHEN MATCHED THEN DELETE

[johanl-db]

What changes were proposed in this pull request?

Fixes a bug in MERGE INTO schema evolution so that MERGE statements that contain a DELETE clause correctly trigger schema evolution:

MERGE INTO target
USING source
ON target.id = source.id
WHEN MATCHED THEN DELETE
WHEN NOT MATCHED THEN INSERT SET *; 

Currently, schemaEvolutionReady always return false when there is a DELETE clause.

How was this patch tested?

Added a test, confirmed that the test fails without this fix.

[johanl-db]

@aokolnychyi / @cloud-fan this is ready to merge

[cloud-fan]

LGTM, thanks!

[cloud-fan]

thanks, merging to master!