Fix more nullability errors

src/Umbraco.Core/Models/IDataValueEditor.cs

@@ -40,7 +40,7 @@ namespace Umbraco.Cms.Core.Models
         /// <param name="value">The property value.</param>
         /// <param name="required">A value indicating whether the property value is required.</param>
         /// <param name="format">A specific format (regex) that the property value must respect.</param>
-        IEnumerable<ValidationResult> Validate(object value, bool required, string format);
+        IEnumerable<ValidationResult> Validate(object value, bool required, string? format);
 
         /// <summary>
         /// Gets the validators to use to validate the edited value.