Permit an absent preferences pillar data key

apt/map.jinja

@@ -11,6 +11,7 @@
         'remove_sources_list': false,
         'clean_sources_list_d': false,
         'preferences_dir': '/etc/apt/preferences.d',
+        'preferences': {},
         'remove_preferences': false,
         'clean_preferences_d': false,
         'default_keyserver': 'pool.sks-keyservers.net',