Improve handling of missing SDK prompts
Improve handling of missing SDK prompts

This fixes #1795 but not showing an SDK prompt if we don't detect any type of project (eg. if VS Code activates the extension because it was just installed). It also ensures if you run a command like `Flutter Doctor` when this happens, it will then show a prompt.

Dart-Code/Dart-Code,Dart-Code/Dart-Code,Dart-Code/Dart-Code,Dart-Code/Dart-Code,Dart-Code/Dart-Code