validation client-side: Fix Required(AllowEmptyString=true) #2894

Job	Run time
Build published projects without warnings (Release)	2m 44s
.NET unit tests (ubuntu-latest)	3m 2s
Build published projects without warnings (Debug)	2m 49s
Build all projects without errors	6m 57s
.NET unit tests (windows-2022)	9m 9s
.NET unit tests (macOS-latest)	5m 45s
UI tests (chrome, ubuntu-latest, Production, Default)	13m 48s
UI tests (chrome, windows-2022, Development, Default)	31m 45s
UI tests (firefox, ubuntu-latest, Development, Default)	19m 0s
UI tests (firefox, ubuntu-latest, Production, Default)	17m 42s
JS unit tests	1m 13s
UI tests (firefox, ubuntu-latest, Development, ExperimentalFeaturesTests)	18m 30s
UI tests (chrome, ubuntu-latest, Production, ExperimentalFeaturesTests)	14m 36s
	2h 27m 0s

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