The asset that this asset is a proxy for, or null if it is not a proxy for another asset.
Note that even if this asset is acting as a proxy for another asset, but this asset is not the default
proxy, then proxy-target will *still* point to this other asset. So you
should check the proxy property of target-proxy before assuming it is the
current default proxy for the target.
Note that the notify for this property is emitted after the proxy notify for the corresponding (if any) asset it is now the proxy of/no longer the proxy of.