use same getattr pattern in the XModule constructor that exists in the XModuleDescriptor

common/lib/xmodule/xmodule/x_module.py

@@ -173,11 +173,11 @@ class XModule(XModuleFields, HTMLSnippet, XBlock):
             # don't need to set category as it will automatically get from descriptor
         elif 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()