NativeJsonInput: treat object fields with `undefined` value as missing

[pkurcx]

Problem

NativeJsonInput (scala-commons, com.avsystem.commons.serialization.nativejs) only treats a missing key in a js.Dictionary / js.Object as "no value". A key that is present but set to undefined is reported as a real
field, which then fails to decode — typically with a ReadFailure — instead of being treated as absent.

This is surprising in a JS context: in JavaScript, { foo: undefined } and {} are effectively equivalent for property reads (obj.foo === undefined in both cases), and many JS-side producers emit undefined for optional
fields rather than omitting the key. As a result, payloads that round-trip through JS code (browser state, js.Dictionary built incrementally, libraries that set obj.foo = undefined for "no value") break deserialization
for fields that should be optional / have defaults.

Reproduction

val dict = js.Dictionary[js.Any]("foo" -> js.undefined)
val input = new NativeJsonInput(dict.asInstanceOf[js.Any], NativeFormatOptions())
GenCodec.read[CaseClassWithOptionalFoo](input) // fails on "foo"

vs. the working case:

val dict = js.Dictionary[js.Any]() // "foo" not set at all
// reads fine — `foo` falls back to default / Opt.Empty

Expected

NativeJsonInput should treat field: undefined the same as a missing field:

1. ObjectInput.hasNext / nextField should skip entries whose value is js.isUndefined.
2. ObjectInput.peekField(name) should return Opt.Empty when the key is present but its value is undefined, in addition to the key-not-present case.