mssmt: guard nil children in BranchNode.Copy

[jtobin]

(This was sussed out by #2117, which will require the fix in order for its CI to pass.)

An MS-SMT's BranchNode, which always has two children (it's a branch), has two valid representations:

1. a full branch, with children materialised, as produced by the in-memory Golang store via pointer traversal, and
2. a so-called "computed branch," with hash/sum precomputed, but children not yet computed (i.e. they're in the database, or certainly should be, but we haven't fetched them yet), as produced by the SQL backends via one-level-at-a-time fetch.

Copy() should handle both, but previously it assumed the full branch / in-memory representation and dereferenced children unconditionally. That caused panics when using e.g. sqlite3 and postgres backends for the tree store, since the children in the computed branch representation are always nil.

The fix is to just do an explicit nil check, dereferencing child nodes only if they're non-nil. This ensures the BranchNode is faithfully copied, whatever its representation (if we're copying a computed branch, the copy will also be a computed branch).

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical issue in the BranchNode.Copy() method within the MS-SMT implementation, preventing panics that occurred when copying "computed branches" where child nodes are intentionally nil. By introducing defensive nil checks, the change ensures that the Copy() operation is robust across different branch representations, particularly those used by SQL backends, thereby improving the stability and reliability of the tree store.

Highlights

• Fixes Panic in BranchNode.Copy(): Addressed an issue where BranchNode.Copy() would panic when encountering nil child nodes, specifically in "computed branch" representations where children are not yet materialized.
• Introduces Nil Checks: Implemented explicit nil checks for Left and Right child nodes before attempting to access their NodeHash() and NodeSum() methods.
• Ensures Correct Copying of Computed Branches: The Copy() method now correctly handles both full and computed branch representations, preserving the nil state of unmaterialized children and preventing crashes with SQL backends.

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[gemini-code-assist]

Code Review

This pull request updates the Copy method for BranchNode to include nil checks for child nodes, preventing potential null pointer dereferences. The reviewer suggests refactoring the logic for creating computed nodes to reduce duplication and improve readability, noting that the current implementation can be simplified.

[lightninglabs-deploy]

@Roasbeef: review reminder
@jtobin, remember to re-request review from reviewers when ready