Bug when deleting parent record, CascadeDelete is set to false and was supposed to clear the reference to null

[burningice2866]

Consider this datatype

public interface ISubscriptionPayment : IData
{
    [StoreFieldType(PhysicalStoreFieldType.Integer)]
    [ImmutableFieldId("ce93accd-2892-4407-ba3c-1d899a93c875")]
    int Id { get; set; }

    [StoreFieldType(PhysicalStoreFieldType.Integer, IsNullable = true)]
    [ImmutableFieldId("33ae5a3a-5155-481c-abe5-4e2ee7a6e806")]
    [ForeignKey(typeof(ISubscriptionSignup), nameof(ISubscriptionSignup.SubscriptionId), AllowCascadeDeletes = false, NullReferenceValue = null)]
    int? SubscriptionId { get; set; }
}

When i delete the parent ISubscriptionSignup record, i expect the delete to happen successfully and SubscriptionId on all child records to be set to null.

Instead i get the error One of the given data items is referenced by one or more data items that do not allow cascade delete. and after some debugging through the callstacks i end up here

C1-CMS-Foundation/Composite/Data/DataFacadeImpl.cs

Lines 515 to 521 in 5910700

	foreach (var refereeInfo in notOptionalReferences)
	{
	if (!refereeInfo.Item2.AllowCascadeDeletes)
	{
	throw new InvalidOperationException("One of the given data items is referenced by one or more data items that do not allow cascade delete.");
	}
	}

and here

C1-CMS-Foundation/Composite/Data/ForeignPropertyInfo.cs

Lines 108 to 111 in 5910700

	public bool IsOptionalReference
	{
	get { return IsNullableString || SourcePropertyInfo.PropertyType == typeof (Guid?); }
	}

.

Clearly the OptionalReference is being resolved wrong and naively expects that such thing can only happen using string or nullable guids.

The correct way would to look at any nullable type, not only guid.