Update SMCalloutView to fix build warnings

[friedbunny]

We’ll want to wait until nfarina/calloutview#107 is merged, which eliminates build warnings we've been seeing.

There haven’t been many other changes to SMCalloutView in the last year, but here are the significant ones:

• nfarina/calloutview@3ae261c — adds nullability annotations to most methods. (Unfortunately conflated with doc changes.)
• nfarina/calloutview@4cb9827 — Fixes colliding animations.
• nfarina/calloutview@2ee6b81 — Minor nullability fix.

/cc @1ec5