Add opt-in for converters to handle null

[layomia]

Fixes #34439.

[ghost]

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[Dotnet-GitSync-Bot]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[layomia]

This change does not regress existing benchmarks.

dotnet run --base "D:\benchmarks\handle_null\before" --diff "D:\benchmarks\handle_null\after" --threshold 3%
summary:
better: 1, geomean: 1.061
worse: 2, geomean: 1.036
total diff: 3

Slower	diff/base	Base Median (ns)	Diff Median (ns)	Modality
System.Text.Json.Serialization.Tests.WriteJson.Serial	1.04	692.62	717.90
System.Text.Json.Serialization.Tests.WriteJson.Seria	1.04	349.66	362.06

Faster	base/diff	Base Median (ns)	Diff Median (ns)	Modality
System.Text.Json.Serialization.Tests.WriteJson.SerializeToString	1.06	410.06	386.40

before.zip
after.zip