have XModuleDescriptor properly ensure category is set

common/lib/xmodule/xmodule/video_module.py

@@ -215,10 +215,6 @@ class VideoDescriptor(VideoFields, TabsEditingDescriptor, EmptyDataRawDescriptor
             self._model_data.update(model_data)
             del self.data
 
-        # I'm not sure how, but in some lifecycles we can loose the .category attribute
-        # this needs to be investigated
-        self.category = 'video'
-
     @classmethod
     def from_xml(cls, xml_data, system, org=None, course=None):
         """

common/lib/xmodule/xmodule/x_module.py

@@ -467,11 +467,11 @@ class XModuleDescriptor(XModuleFields, HTMLSnippet, ResourceTemplates, XBlock):
         self.system = self.runtime
         if isinstance(self.location, Location):
             self.url_name = self.location.name
-            if not hasattr(self, 'category'):
+            if getattr(self, 'category', None) is None:
                 self.category = self.location.category
         elif isinstance(self.location, BlockUsageLocator):
             self.url_name = self.location.usage_id
-            if not hasattr(self, 'category'):
+            if getattr(self, 'category', None) is None:
                 raise InsufficientSpecificationError()
         else:
             raise InsufficientSpecificationError()