Fix reviewer roulette when no-one is in a category

This would return `people.size` before, when it should return `nil`.

danger/roulette/Dangerfile

@@ -57,10 +57,12 @@ def spin_for_person(people, random:)
   people.size.times do
     person = people.sample(random: random)
 
-    return person unless out_of_office?(person)
+    break person unless out_of_office?(person)
 
     people -= [person]
   end
+
+  person
 end
 
 def out_of_office?(person)