Fix activation of unknown apps

Fixes error messages like this:
````
{"app":"PHP","message":"Invalid argument supplied for foreach() at \/home\/deepdiver\/Development\/ownCloud\/core\/lib\/private\/installer.php#413","level":2,"time":"2014-01-15T15:59:03+00:00"}
{"app":"PHP","message":"Invalid argument supplied for foreach() at \/home\/deepdiver\/Development\/ownCloud\/core\/lib\/private\/installer.php#416","level":2,"time":"2014-01-15T15:59:03+00:00"}
{"app":"PHP","message":"An exception occurred while executing 'INSERT INTO \"oc_appconfig\" ( \"appid\", \"configkey\", \"configvalue\" ) VALUES( ?, ?, ? )':\n\nSQLSTATE[23000]: Integrity constraint violation: 19 oc_appconfig.appid may not be NULL at \/home\/deepdiver\/Development\/ownCloud\/core\/3rdparty\/doctrine\/dbal\/lib\/Doctrine\/DBAL\/DBALException.php#47","level":3,"time":"2014-01-15T15:59:03+00:00"}