[Fix] stringify: skip null/undefined filter-array entries instead of crashing in encoder - #551

…f crashing in `encoder`

When `filter` is an array, its elements become `objKeys` directly and the top-level loop passes each entry as the recursive `prefix` argument without coercion.
A `null` or `undefined` entry reached `utils.encode` as the `str` argument and `str.length` threw `TypeError`.

The crash requires two conditions to align: the filter array must contain `null`/`undefined`, and the object being stringified must have a literal `'null'`/`'undefined'` property
(otherwise `obj[key]` is `undefined` and the typeof-undefined early-return at stringify.js:137 fires before the encoder is called)

Both are reachable from an attacker controlled input in applications that stringify user-submitted JSON with user-selected fields.

Fix this up by skipping `null`/`undefined` filter entries at the top-level loop.
This somewhat matches `JSON.stringify` replacer-array semantics, which silently ignore non-string/non-number entries.
`Object.keys` never yields `null`/`undefined`, so the guard only affects user-supplied filter arrays.
The inner recursive loop already coerces `key` via `String()` when building `keyPrefix`, so it never passes a raw `null` to the encoder.