fix(commitTreeBuilder): crash when parentNode.children is undefined

[Mohammadreza-Sarvari]

Closed #36596

Dear @unstubbable @eps1lon could you please assign #36596 issue to me and review this PR? thanks.

Summary

Guard against undefined children array in CommitTreeBuilder when calling .concat() and .filter() on cloned parent nodes. This fixes a crash in the Profiler flamegraph view: "Cannot read properties of undefined (reading 'concat')" in CommitFlamegraphAutoSizer.
The root cause: getClonedNode spreads an existing node ({...existingNode}), and in some edge cases the node's children field can be undefined, causing the subsequent .concat() or .filter() call to throw.

How did you test this change?

1. Reproduced the original crash in the DevTools Profiler flamegraph view.
2. Verified the fix by applying the guard (parentNode.children || []) at both call sites (.concat() at line 258 and .filter() at line 308).
3. Confirmed no other unguarded .children access in the same file that could cause the same class of error.

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