fix(json-crdt): guard ObjNode.children against missing nodes

[privateJiangyaokai]

Problem

ObjNode.children() passes the result of index.get(id) straight to the callback with a non-null assertion:

public children(callback: (node: JsonNode) => void) {
  const index = this.doc.index;
  this.keys.forEach((id, key) => callback(index.get(id)!));
}

index.get(id) can legitimately return undefined when a key references a node id that is not present in the model index (e.g. a dangling reference encountered while applying a patch). The ! assertion hides this, so undefined reaches the callback.

The most visible symptom is a crash in Model._gcTree (formerly deleteNodeTree), which reads value.sid on its argument before the index.get existence check:

_gcTree(value: ITimestampStruct) {
  const isSystemNode = value.sid === 0;  // TypeError: Cannot read properties of undefined (reading 'sid')
  ...
  node.children((child) => this._gcTree(child.id));
}

When an ObjNode has a child key pointing at a missing node, children() invokes the callback with undefined, and _gcTree(undefined) throws Cannot read properties of undefined (reading 'sid'), aborting the whole patch/GC pass.

Fix

Guard against the missing node before invoking the callback — exactly what ArrNode.children() and VecNode.children() already do:

public children(callback: (node: JsonNode) => void) {
  const index = this.doc.index;
  this.keys.forEach((id) => {
    const node = index.get(id);
    if (node) callback(node);
  });
}

This makes ObjNode consistent with the other collection nodes:

• ArrNode.children: const node = index.get(data[i]); if (node) callback(node);
• VecNode.children: const node = index.get(id); if (node) callback(node);

Only ObjNode was missing the guard (and additionally used an unsound !). No behavior change for well-formed models; it only prevents a hard crash when a dangling reference is present.