Honor registered json converters that has `HandleNull = true`

[rmja]

The current ConversionResultProvider backs out if it sees value null for non-reference types. I explicitly have a registered a json converter to handle this type, and it has HandleNull = true. I would expect this converter to be called in this case, instead of backing out and failing the conversion.

[rmja]

Related issue: dotnet/aspnetcore#65182