Specify nullability for drop-in compatibility from Swift

[samsonjs]

Before this change SDAVAssetExportSession was not actually a drop-in replacement for AVAssetExportSession when called from Swift.

One concession I made is that the initializer return values are nullable even though we don't ever return nil, purely for compatibility with AVAssetExportSession. Given other differences in the initializer method signature maybe this isn't the right call. It's certainly open to debate.