Courses having 'null' in course-end date shows incorrect course card

Course-discovery is returning incorrect availability status of the
courses that have 'null' value in their course_end date.To avoid it,
a conditional statement is modified so that it will behave as expected.

LEARNER-3218

course_discovery/apps/course_metadata/models.py

@@ -724,7 +724,7 @@ class CourseRun(TimeStampedModel):
 
         if self.end and self.end <= now:
             return _('Archived')
-        elif self.start and self.end and (self.start <= now < self.end):
+        elif self.start and (self.start <= now):
             return _('Current')
         elif self.start and (now < self.start < upcoming_cutoff):
             return _('Starting Soon')

course_discovery/apps/course_metadata/tests/test_models.py

@@ -162,6 +162,7 @@ class CourseRunTests(TestCase):
         ('2030-01-01 00:00:00Z', None, 'Upcoming'),
         (None, '2016-01-01 00:00:00Z', 'Archived'),
         ('2015-01-01 00:00:00Z', '2016-01-01 00:00:00Z', 'Archived'),
+        ('2015-10-24 00:00:00Z', None, 'Current'),
         ('2016-01-01 00:00:00Z', '2017-01-01 00:00:00Z', 'Current'),
         ('2016-07-21 00:00:00Z', '2017-01-01 00:00:00Z', 'Starting Soon'),
     )