Fix CS8618 warnings

Code CS1591: Non-nullable field must contain a non-null value when exiting constructor.

Author: Nirmal4G

UnitTests/UnitTests.NetCore/Mvvm/Test_ObservablePropertyAttribute.cs

@@ -235,15 +235,15 @@ public enum Animal
         public partial class ModelWithValueProperty : ObservableObject
         {
             [ObservableProperty]
-            private string value;
+            private string? value;
         }
 
         public partial class ModelWithValuePropertyWithValidation : ObservableValidator
         {
             [ObservableProperty]
             [Required]
             [MinLength(5)]
-            private string value;
+            private string? value;
         }
     }
 }