Revert "Fixed #20956 -- Removed useless check in django.db.utils"

This reverts commit ce0e86c.

The check is necessary if 'ENGINE' is an empty string.
Thanks apollo13 for pointing this out.

django/db/utils.py

@@ -173,7 +173,7 @@ def ensure_defaults(self, alias):
             conn.setdefault('AUTOCOMMIT', False)
         conn.setdefault('AUTOCOMMIT', True)
         conn.setdefault('ENGINE', 'django.db.backends.dummy')
-        if conn['ENGINE'] == 'django.db.backends.':
+        if conn['ENGINE'] == 'django.db.backends.' or not conn['ENGINE']:
             conn['ENGINE'] = 'django.db.backends.dummy'
         conn.setdefault('CONN_MAX_AGE', 0)
         conn.setdefault('OPTIONS', {})