onChunk property Null safety

Callback for getting notified when a chunk of bytes has been received during the loading of the image.

This callback may fire many times (e.g. when used with a NetworkImage, where the image bytes are loaded incrementally over the wire) or not at all (e.g. when used with a MemoryImage, where the image bytes are already available in memory).

This callback may also continue to fire after the onImage callback has fired (e.g. for multi-frame images that continue to load after the first frame is available).