Error thrown in cycle analytics events if build has not started

We'll have to deal with builds that have not yet started

ArgumentError at /gitlab-org/gitlab-ce/cycle_analytics/events/test.json
=======================================================================

> comparison of Time with nil failed

app/serializers/entity_date_helper.rb, line 5
---------------------------------------------


    1   module EntityDateHelper
    2     include ActionView::Helpers::DateHelper
    3   
    4     def interval_in_words(diff)
>   5       "#{distance_of_time_in_words(Time.now, diff)} ago"
    6     end
    7   
    8     # Converts seconds into a hash such as:
    9     # { days: 1, hours: 3, mins: 42, seconds: 40 }
   10     #

App backtrace
-------------

 - app/serializers/entity_date_helper.rb:5:in `interval_in_words'
 - app/serializers/analytics_build_entity.rb:12:in `block in <class:AnalyticsBuildEntity>'
 - lib/gitlab/cycle_analytics/staging_event.rb:27:in `serialize'
 - lib/gitlab/cycle_analytics/base_event.rb:18:in `block in fetch'
 - lib/gitlab/cycle_analytics/base_event.rb:17:in `fetch'
 - lib/gitlab/cycle_analytics/staging_event.rb:17:in `fetch'
 - lib/gitlab/cycle_analytics/events.rb:22:in `test_events'
 - app/controllers/projects/cycle_analytics/events_controller.rb:26:in `test'
 - lib/gitlab/request_profiler/middleware.rb:15:in `call'
 - lib/gitlab/middleware/go.rb:16:in `call'
 - lib/gitlab/middleware/static.rb:9:in `call'