Fix run value converters that convert nulls for JSON columns

[JoasE]

During (de)serialization, check if converter converts nulls and run for null values if so

Closes: #37983

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        Fixes #bugnumber

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[JoasE]

Unfortunately my github copilot has reached it's token limit, so I can't request review's anymore until next week. If you assign copilot I will fix the issues, or I'll wait till next week

[JoasE]

This is copied over from above for CreateGetValueExpression

[roji]

@AndriySvyryd assigning to you for reviewing but let me know if you want me to do it.

[Copilot]

Pull request overview

Fixes JSON column (de)serialization so value converters that opt into null conversion (ConvertsNulls == true) are applied even when the JSON token/value is null, addressing incorrect null persistence/materialization behavior.

Changes:

• Apply JSON value converter logic for null values during JSON update serialization (ModificationCommand, RelationalJsonUtilities).
• Apply value converter logic for JSON null tokens during query materialization (RelationalShapedQueryCompilingExpressionVisitor).
• Add/adjust relational and provider-specific tests to validate null-converting converters in JSON scenarios.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/EFCore.Relational/Update/ModificationCommand.cs	Serialize JSON properties via JsonValueReaderWriter even when the value is null, if the converter converts nulls.
src/EFCore.Relational/Query/Internal/RelationalJsonUtilities.cs	Same as above for JSON parameter/constant serialization utilities.
src/EFCore.Relational/Query/RelationalShapedQueryCompilingExpressionVisitor.ShaperProcessingExpressionVisitor.cs	When JSON token is null, invoke converter (when ConvertsNulls) instead of returning default CLR value.
test/EFCore.Relational.Specification.Tests/Query/AdHocJsonQueryRelationalTestBase.cs	Adds a regression test exercising JSON equality with a converter that converts nulls.
test/EFCore.SqlServer.FunctionalTests/Query/AdHocJsonQuerySqlServerTestBase.cs	Adds SQL assertion for the new regression scenario.
test/EFCore.Specification.Tests/ValueConvertersEndToEndTestBase.cs	Refactors base test to allow provider-specific JSON embedding (new overridable hooks).
test/EFCore.SqlServer.FunctionalTests/ValueConvertersEndToEndSqlServerJsonTest.cs	New SQL Server functional test running the end-to-end converter suite with the entity stored as JSON.
test/EFCore.SqlServer.FunctionalTests/Update/JsonUpdateSqlServerTest.cs	Updates expected JSON payload/parameter sizing for null-converting converter output.
test/EFCore.SqlServer.FunctionalTests/Update/JsonUpdateJsonTypeSqlServerTest.cs	Same as above for json column type.
test/EFCore.Sqlite.FunctionalTests/Update/JsonUpdateSqliteTest.cs	Updates expected JSON payload/parameter sizing for null-converting converter output.

[AndriySvyryd]

propertyValue can be null here

[JoasE]

That’s actually intentional here, since this change allows propertyValue to be null if the converter has indicated it can handle null values.

[AndriySvyryd]

Then you should change the signature of ToJson to allow nulls

[AndriySvyryd]

propertyValue can be null here

[JoasE]

Same as above