Fix provider null return value

[yuwen-yan]

Follow-up PR for #202

[luigi-rosso]

This is cleaner, thank you. I'm curious though, how was the provider returning null prior to these changes?

[yuwen-yan]

@luigi-rosso it's about error handling, AFAIK, there are two ways to fail:

• Throw exception in AssetProvider.load() , in this case, we should raise this exception to client application and show nothing.
• Return invalid ByteData (including null) by AssetProvider.load(), in this case, right now we will raise an exception when exec FlutterActor.loadFromByteData(data).

I was thinking that we should suppress the exception for invalid ByteData, but after a second thought, I think it's also ok to raise it, WDYT? So I'm going to hold this PR for now..