Converter attempts to instantiate an entity if the database value is `null`

[Inego]

Two related problems:

1. In case when a custom converter is provided for a type and a null value is converted, there is no way to disambiguate whether I want it to be be left null or converted to a specific instance of the target type.
   For example, if my target type is Jackson's JsonNode and the souce value is null, it's not possible to customize conversion to return just null or com.fasterxml.jackson.databind.node.NullNode.

2. MappingR2dbcConverter fails to convert null values to JsonNode even in the presence of the corresponding converter, see an example here.

	Suppressed: org.springframework.data.mapping.MappingException: Could not read property private final com.fasterxml.jackson.databind.JsonNode com.example.r2dbcconverternull.MyRow.field1 from column field1!
		at app//org.springframework.data.r2dbc.convert.MappingR2dbcConverter.readFrom(MappingR2dbcConverter.java:185)
		at app//org.springframework.data.r2dbc.convert.MappingR2dbcConverter.access$000(MappingR2dbcConverter.java:64)
		at app//org.springframework.data.r2dbc.convert.MappingR2dbcConverter$RowParameterValueProvider.getParameterValue(MappingR2dbcConverter.java:706)
		at app//org.springframework.data.mapping.model.SpELExpressionParameterValueProvider.getParameterValue(SpELExpressionParameterValueProvider.java:53)
		at app//org.springframework.data.relational.core.conversion.BasicRelationalConverter$ConvertingParameterValueProvider.getParameterValue(BasicRelationalConverter.java:268)
		at app//org.springframework.data.mapping.model.ClassGeneratingEntityInstantiator.extractInvocationArguments(ClassGeneratingEntityInstantiator.java:276)
		at app//org.springframework.data.mapping.model.ClassGeneratingEntityInstantiator$EntityInstantiatorAdapter.createInstance(ClassGeneratingEntityInstantiator.java:248)
		at app//org.springframework.data.mapping.model.ClassGeneratingEntityInstantiator.createInstance(ClassGeneratingEntityInstantiator.java:89)
		at app//org.springframework.data.relational.core.conversion.BasicRelationalConverter.createInstance(BasicRelationalConverter.java:145)
		at app//org.springframework.data.r2dbc.convert.MappingR2dbcConverter.createInstance(MappingR2dbcConverter.java:326)
		at app//org.springframework.data.r2dbc.convert.MappingR2dbcConverter.read(MappingR2dbcConverter.java:124)
		at app//org.springframework.data.r2dbc.convert.MappingR2dbcConverter.read(MappingR2dbcConverter.java:119)
		at app//com.example.r2dbcconverternull.R2dbcConverterNullApplicationTests.convertNullValue$lambda-0(R2dbcConverterNullApplicationTests.kt:33)
		at app//io.r2dbc.h2.H2Result.lambda$map$0(H2Result.java:67)
		at app//reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:106)
		... 181 more
	Caused by: org.springframework.data.mapping.model.MappingInstantiationException: Failed to instantiate com.fasterxml.jackson.databind.JsonNode using constructor protected com.fasterxml.jackson.databind.JsonNode() with arguments 
		at app//org.springframework.data.mapping.model.ClassGeneratingEntityInstantiator$MappingInstantiationExceptionEntityInstantiator.createInstance(ClassGeneratingEntityInstantiator.java:321)
		at app//org.springframework.data.mapping.model.ClassGeneratingEntityInstantiator.createInstance(ClassGeneratingEntityInstantiator.java:89)
		at app//org.springframework.data.relational.core.conversion.BasicRelationalConverter.createInstance(BasicRelationalConverter.java:145)
		at app//org.springframework.data.r2dbc.convert.MappingR2dbcConverter.createInstance(MappingR2dbcConverter.java:326)
		at app//org.springframework.data.r2dbc.convert.MappingR2dbcConverter.readEntityFrom(MappingR2dbcConverter.java:293)
		at app//org.springframework.data.r2dbc.convert.MappingR2dbcConverter.readFrom(MappingR2dbcConverter.java:175)
		... 195 more
	Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.fasterxml.jackson.databind.JsonNode]: Class is abstract
		... 201 more

[mp911de]

That's by design and by intent. We generally do not convert null values. Part of the rationale is that in some cases we do not have sufficient type information (e.g. convert null to Object). What seems to be a bug is that we attempt to create an instance while the database is null. We should fix that.