Prevent nulls inside array/lists

[drhumlen]

You should not accept ["hello", null, "world"] unless the type is string option list. If the type is a string list with nulls inside, it should fail.

This has led to nasty NullReferenceExceptions in our backend because the code downstream expected a list of 'a, but the list turned out to contain some nulls.

I'm currently trying to fix it, but I'm unable to use the isNullableFieldType (from Helpers.fs) because the fsOptions: JsonFSharpOptions isn't avaiable in the type JsonListConverter<'T>() etc classes.

[drhumlen]

#58

[drhumlen]

Merged.