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.

spec/components/admin/budgets/duration_component_spec.rb

@@ -43,6 +43,22 @@ describe Admin::Budgets::DurationComponent, type: :component do
 
       expect(page.text).to eq "about 1 year"
     end
+
+    it "is not defined when no end date is defined" do
+      durable = double(starts_at: Time.zone.local(2015, 8, 1, 12, 0, 0), ends_at: nil)
+
+      render Admin::Budgets::DurationComponent.new(durable).duration
+
+      expect(page.text).to be_empty
+    end
+
+    it "is not defined when no start date is defined" do
+      durable = double(starts_at: nil, ends_at: Time.zone.local(2016, 9, 30, 16, 30, 00))
+
+      render Admin::Budgets::DurationComponent.new(durable).duration
+
+      expect(page.text).to be_empty
+    end
   end
 
   attr_reader :content
@@ -52,6 +68,6 @@ describe Admin::Budgets::DurationComponent, type: :component do
   end
 
   def page
-    Capybara::Node::Simple.new(content)
+    Capybara::Node::Simple.new(content.to_s)
   end
 end