validation client-side: Fix Required(AllowEmptyString=true) #2895

Job	Run time
Build all projects without errors	8m 32s
Build published projects without warnings (Release)	2m 41s
.NET unit tests (ubuntu-latest)	3m 22s
JS unit tests	1m 13s
Build published projects without warnings (Debug)	2m 43s
.NET unit tests (windows-2022)	7m 43s
UI tests (chrome, ubuntu-latest, Production, Default)	12m 50s
.NET unit tests (macOS-latest)	8m 15s
UI tests (chrome, windows-2022, Development, Default)	33m 17s
UI tests (firefox, ubuntu-latest, Development, Default)	18m 52s
UI tests (firefox, ubuntu-latest, Production, Default)	17m 48s
UI tests (firefox, ubuntu-latest, Development, ExperimentalFeaturesTests)	18m 58s
UI tests (chrome, ubuntu-latest, Production, ExperimentalFeaturesTests)	13m 17s
	2h 29m 31s

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