Fix for null string values

[RehanSaeed]

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Fixes:

• https://github.com/RehanSaeed/Schema.NET/issues/389

[Turnerj]

Looks like our issue is this block of code in S.T.J: https://source.dot.net/#System.Text.Json/System/Text/Json/Serialization/Metadata/JsonPropertyInfoOfT.cs,353

There are a couple of flags around there that we might be able to toggle to get this to behave correctly.

[Turnerj]

I should mention - that the block of code I linked to is likely based on the master/main code in the Runtime, not necessarily what we are using. ~We don't have individual control of HandleNullOnRead currently, just a HandleNull property which does read and write.~ (Edit: HandleNullOnRead is internal)

I imagine (though haven't confirmed) is that if we enable HandleNull, it might mean that null/default values are going to be serialized again.

[Turnerj]

Setting HandleNull does make our converter run however another internal part of S.T.J trips:

I'm thinking that S.T.J is doing this because a struct can't technically be null instead of setting the struct to its default value.

[Turnerj]

Tried to mess around with a few settings but doesn't seem like there is anything we can do without trying something drastic like switching Values and OneOrMany to be reference types.