[tools] `AndroidMavenArtifacts` is a no-op if the Android SDK is unavailable, but it is unclear if it should be

Discovered while implementing #127354

https://github.com/flutter/flutter/blob/e8f5a51f79fc5dd2f8de38b4d9f504d09dca8197/packages/flutter_tools/lib/src/flutter_cache.dart#L411-L413

It is unclear if this null guard should be here.