StoreGate+AsgDataHandles: Better handling for empty decoration name.

We allow initializing a WriteDecorHandleKey from another HandleKey and a decoration name. In that case, the container name part of the full decoration key is taken from the other handle.

But in the case of an optional decoration, we allow the key to be set to a null string, initialized with SG::AllowEmpty, and then tested with empty().

These didn't interact properly. If the decoration name was empty, we would still get the container name in the key, causing it not to be considered as empty. Fixed.

Also make the same changes in AsgDataHandles.