Preserve reviewed deserialized JsonDocument null guards

Preserve a legacy null branch when .NET 9 deserializes JSON null into a non-null JsonDocument. Admit only a typed JsonSerializer.Deserialize<JsonDocument> using declaration immediately followed by a null-only throwing if guard on that same local. Extend the condition to recognize JsonValueKind.Null; keep the original call, options, branches and using disposal. Requires explicit converter-policy review. Skip nonthrowing guards, guards observing the local, captures/async/ref scopes, other producers, ordinary locals, intervening statements, unresolved symbols and conditional snapshots. Standalone opt-in; do not apply to a project intentionally adopting the new null-document behavior.