Add NullablePtr type

[stevenfontanella]

No description provided.

[kripken]

Curious to hear more opinions here, but I generally feel that we can use default values of =nullptr in such cases where null is possible. I mean

Suggested change

	NullablePtr<const EffectAnalyzer*> bodyEffects) {
	const EffectAnalyzer* bodyEffects=nullptr) {

This generally what we've done so far.

And, on the other hand, we do have many places with a pointer that is non-nullable, but I wonder if the best thing is not to convert those to references. Though it would be a large change.