Fix null annotation of ViewModelPropertyMap.SetJsonConverter #3997

Job	Run time
Build published projects without warnings (Release)	3m 9s
CommandLine tests (ubuntu-latest)	3m 13s
JS unit tests	54s
CommandLine tests (macOS-latest)	3m 5s
CommandLine tests (windows-2022)	5m 55s
Build published projects without warnings (Debug)	3m 16s
UI tests (chrome, windows-2022, Development, Default)	18m 20s
.NET unit tests (windows-2022)	8m 50s
UI tests (chrome, ubuntu-latest, Production, ExperimentalFeaturesTests)	13m 51s
.NET unit tests (ubuntu-latest)	7m 5s
UI tests (firefox, ubuntu-latest, Production, Default)	13m 33s
.NET unit tests (macOS-latest)	4m 37s
UI tests (chrome, ubuntu-latest, Production, Default)	13m 37s
UI tests (firefox, ubuntu-latest, Development, Default, /test-basepath)	14m 32s
Build all projects without errors	8m 1s
UI tests (firefox, ubuntu-latest, Development, ExperimentalFeaturesTests)	14m 45s
	2h 16m 43s

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