Possible NullReferenceException in MultiUrlPickerValueConverter (#12109)

* Fixed a bug in the MultiUrlPickerValueConverter file, where an invalid value being set will result in a NullReferenceException

* Change to checking for whitespace values before de-serializing

Co-authored-by: Paul Woodland <paul.woodland@pwnewmedia.com>

src/Umbraco.Infrastructure/PropertyEditors/ValueConverters/MultiUrlPickerValueConverter.cs

@@ -51,7 +51,7 @@ namespace Umbraco.Cms.Core.PropertyEditors.ValueConverters
             {
                 var maxNumber = propertyType.DataType.ConfigurationAs<MultiUrlPickerConfiguration>().MaxNumber;
 
-                if (inter == null)
+                if (string.IsNullOrWhiteSpace(inter?.ToString()))
                 {
                     return maxNumber == 1 ? null : Enumerable.Empty<Link>();
                 }