Fix null handling when calling custom JsonConverters

[exyi]

System.Text.Json converters have HandleNull property, which we should respect:

• when false (default), both serialization and deserialization should handle null values before the converter is called. Conterintuitively this is the case even if converter is explicitely defined for Nullable<...> type
• when true: we don't check for null and let converter handle it

We always allow null in DotvvmConverters, because they all already handle it, and it keeps the generated code smaller.