Set Cn to identity instead of nil, when deleting a 128-leaf subtree

[gballet]

Even though verkle doesn't support deletions during transaction execution (yet) we have to support deletions so that geth can rollback state during a reorg.

When either the left or right suffix subtree of a LeafNode becomes empty, the commitment must be set to 0. It used to be set to nil, but this would cause panics later down the line when inserting value and trying to differentially-update the commitment.

I can foresee that this will cause an issue for storage size, as we'd potentially be storing a 0. But this will unblock Gary for his rollback experiments.

[Copilot]

Copilot reviewed 1 out of 1 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

tree.go:1387

• Consider adding unit tests to verify that delete operations correctly update both c1 and c2 to the identity point instead of nil, ensuring that rollback behavior works as expected.

n.c1 = new(Point).SetIdentity()

[jsign]

The new changes are aligned on how NewLeafNode works, right? Look here.

Looks like in that else we aren't doing else if count > 0 {, which means cfg.CommitToPoly is always called (apart from the optimized case of emptyCodeHash) which never return nil.

So maybe this means the current change is a more reasonable action than what we had before this PR?