Quote string scalars that resolve to null (#493)

[gaoflow]

A string equal to a null token (~, null, Null, NULL) is emitted as a plain scalar, so it deserializes back to null even when the target is string — the serializer emits output its own deserializer reads as a different type (#493).

var s = new SerializerBuilder().Build();
var d = new DeserializerBuilder().Build();
d.Deserialize<string>(s.Serialize("null")); // returns null, not "null"

TypeAssigningEventEmitter now quotes a string scalar whose plain form matches the exact set NullNodeDeserializer treats as null, so it round-trips. Only that set is quoted — nUll/nullish stay plain — and actual nulls are unaffected (still emit bare null). It is the same set WithQuotingNecessaryStrings already quotes, applied unconditionally for the null tokens because, unlike bool/number look-alikes, they lose data even for a string target.

Tested: the four tokens round-trip as scalars and dictionary values, look-alikes stay unquoted, and real null still emits bare.