Include empty segments in subscribers filters

Although it doesn't make senses to select a segment with 0 subscribers
directly from the dropdown, we allow users to click a "view subscribers"
link or, with MAILPOET-4244, click a badge to see all the subscribers of
 that segment. The UI on the subscribers page indicates which segment is
  selected by selecting it in the dropdown.

 If we exclude lists with 0 subscribers (or 0 calculated
 subscribers), the filter selection is blank and the user has no way of
 knowing what filter is being applied. By including these empty lists
 we're giving users a way of knowing why there are 0 subscribers being
 shown.

MAILPOET-4244

mailpoet/lib/Subscribers/SubscriberListingRepository.php

@@ -271,9 +271,6 @@ class SubscriberListingRepository extends ListingRepository {
       } else {
         $count = $this->subscribersCountsController->getSegmentStatisticsCount($segment);
       }
-      if (!$count[$key]) {
-        continue;
-      }
 
       $segmentList[] = [
         'label' => sprintf('%s (%s)', $segment->getName(), number_format((float)$count[$key])),