fix for BlendMode.isRegistered function error when calling it early

[MatseFR]

if you call this when your main Starling class is created or added to stage, it results in an error because sBlendModes is null. Other BlendMode methods check for this and call registerDefaults() if needed so I did the same

[PrimaryFeather]

Ah, that's a good find — thanks for the quick fix, Mathieu! 🙏