libs/dyn/merge: clear an inherited value when an override is null

[apoorva-01]

Changes

An explicit null in an override now clears the inherited value instead of being ignored, so autoscale: null in a target actually drops the base autoscale block.

I kept it scoped to map entries in merge.Merge (key present + incoming nil = clear). Absent keys aren't affected and top-level Merge(x, nil) == x still holds. The tidier alternative is making nil always win in merge() itself, but that flips a documented invariant three tests assert, and touches every caller. Happy to go that way if you'd rather.

Why

Closes #5701. There's currently no way to drop an inherited field in a target, so you can't switch a job cluster to single-node for just one target.

Tests

Unit test for a null override clearing an existing entry (checks value and locations), plus a no_autoscale target in the override/clusters acceptance test. Both fail without the fix.

[github-actions]

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Inputs:

• PR number: 5949
• Commit SHA: 6202847265647cc657ce68f4a17b4d733df56693

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[github-actions]

Approval status: pending

/acceptance/bundle/ - needs approval

Files: acceptance/bundle/override/clusters/databricks.yml, acceptance/bundle/override/clusters/output.txt, acceptance/bundle/override/clusters/script
Suggested: @janniklasrose
Also eligible: @pietern, @denik, @shreyas-goenka, @andrewnester, @anton-107, @lennartkats-db

General files (require maintainer)

Files: .nextchanges/bundles/merge-null-target-override.md, libs/dyn/merge/merge.go, libs/dyn/merge/merge_test.go
Based on git history:

• @janniklasrose -- recent work in .nextchanges/bundles/

_{Any maintainer (@andrewnester, @anton-107, @denik, @pietern, @shreyas-goenka, @simonfaltum, @renaudhartert-db, @janniklasrose, @lennartkats-db) can approve all areas.
See OWNERS for ownership rules.}