Properly serializing Course/CourseRun level type

This ensures the level type for Courses, and their related CourseRuns, without a specified level type is serialized as None instead of an empty array.

ECOM-5195

course_discovery/apps/api/tests/test_serializers.py

@@ -467,13 +467,9 @@ class CourseRunSearchSerializerTests(TestCase):
 
     def test_data(self):
         course_run = CourseRunFactory()
+        serializer = self.serialize_course_run(course_run)
         course_run_key = CourseKey.from_string(course_run.key)
 
-        # NOTE: This serializer expects SearchQuerySet results, so we run a search on the newly-created object
-        # to generate such a result.
-        result = SearchQuerySet().models(CourseRun).filter(key=course_run.key)[0]
-        serializer = CourseRunSearchSerializer(result)
-
         expected = {
             'transcript_languages': [self.serialize_language(l) for l in course_run.transcript_languages.all()],
             'short_description': course_run.short_description,
@@ -498,6 +494,18 @@ class CourseRunSearchSerializerTests(TestCase):
         }
         self.assertDictEqual(serializer.data, expected)
 
+    def serialize_course_run(self, course_run):
+        """ Serializes the given `CourseRun` as a search result. """
+        result = SearchQuerySet().models(CourseRun).filter(key=course_run.key)[0]
+        serializer = CourseRunSearchSerializer(result)
+        return serializer
+
+    def test_data_without_serializers(self):
+        """ Verify a null `LevelType` is properly serialized as None. """
+        course_run = CourseRunFactory(course__level_type=None)
+        serializer = self.serialize_course_run(course_run)
+        self.assertEqual(serializer.data['level_type'], None)
+
 
 class ProgramSearchSerializerTests(TestCase):
     def test_data(self):

course_discovery/apps/course_metadata/search_indexes.py

@@ -52,7 +52,7 @@ class BaseCourseIndex(OrganizationsMixin, BaseIndex):
     organizations = indexes.MultiValueField(faceted=True)
     authoring_organizations = indexes.MultiValueField(faceted=True)
     sponsoring_organizations = indexes.MultiValueField(faceted=True)
-    level_type = indexes.CharField(model_attr='level_type__name', null=True, faceted=True)
+    level_type = indexes.CharField(null=True, faceted=True)
     partner = indexes.CharField(model_attr='partner__short_code', null=True, faceted=True)
 
     def prepare_subjects(self, obj):
@@ -67,6 +67,9 @@ class BaseCourseIndex(OrganizationsMixin, BaseIndex):
     def prepare_sponsoring_organizations(self, obj):
         return self._prepare_organizations(obj.sponsoring_organizations.all())
 
+    def prepare_level_type(self, obj):
+        return obj.level_type.name if obj.level_type else None
+
 
 class CourseIndex(BaseCourseIndex, indexes.Indexable):
     model = Course