Update to the Propel::getConfiguration() method

Update to the Propel::getConfiguration() method to avoid: "Fatal error: Call to a member function getParameters() on a non-object".
This error occurs because we didn't call Propel::setConfiguration() before calling Propel::getConfiguration().
In my opinion this function should return null (the default value of the configuration class property) instead of an untreated error (fatal error).