Fix crash with budgets with no published phases

In this case, the duration of the budget cannot be determined, and the
application was crashing when trying to do so.

Now we're just returning `nil` as duration.

app/components/admin/budgets/duration_component.rb

@@ -10,7 +10,7 @@ class Admin::Budgets::DurationComponent < ApplicationComponent
   end
 
   def duration
-    distance_of_time_in_words(durable.starts_at, durable.ends_at)
+    distance_of_time_in_words(durable.starts_at, durable.ends_at) if durable.starts_at && durable.ends_at
   end
 
   private