fix: handle custom classes with null proto as pojo#95

There is a common pattern that libraries like [ufo](https://github.com/unjs/ufo/blob/5b87427f05d0f34f484b1c9559175d65573b8e1c/src/query.ts#L21-L25) and [cookie](https://github.com/jshttp/cookie/blob/2a1a4d8b2679b208f354e848e711dc8471fb83af/src/index.ts#L68-L72) use to have a custom class with `null` prototype instead of `Object.create(null)` to improve (v8) [performance](https://jsbenchmark.com/#eyJjYXNlcyI6W3siaWQiOiJQdEZfRWZzQzFVWjRaTXBTU1BWTG4iLCJjb2RlIjoiQXJyYXkuZnJvbSh7bGVuZ3RoOiAxMDAwMH0sICgpID0-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). This PR fixes the devalue behavior to support this pattern. Since there is no obvious way to distinguish `{}` and `new NullProto()` (both have prototype of `[Object: null prototype]`), they are serialized as normal object (with prop oveerrides)