Make wait noop if scheduled_task is nil

This can happen when the Debouncer has finished its work and we clear it
from the current thread. The next call to refresh_debouncer_for creates
a new Debouncer, but it doesn't have a scheduled_task yet.

We only use the wait method in tests, to ensure that the debounces has
finished its work. But if the scheduled_task is nil, we know that the
debouncer has already finished its work.

app/models/turbo/debouncer.rb

@@ -14,7 +14,7 @@ def debounce(&block)
   end
 
   def wait
-    scheduled_task.wait(wait_timeout)
+    scheduled_task&.wait(wait_timeout)
   end
 
   private