Fix check for default app to succeed even if proxy (#3482)

celery/fixups/django.py

@@ -46,7 +46,7 @@ class DjangoFixup(object):
 
     def __init__(self, app):
         self.app = app
-        if default_app is None:
+        if not default_app:
             self.app.set_default()
         self._worker_fixup = None