ruff

c79effff - [ty] Lazily initialize `UnionBuilder` when transforming a union type (#24929)

[ty] Lazily initialize `UnionBuilder` when transforming a union type (#24929) ## Summary This early return improves performance. I tried the same thing at intersections, but there wasn't much change. It seems `UnionType::map_leave_aliases` was a particularly hotspot. ## Test Plan --------- Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>