The check whether there is an old configuration always returns true, even for new installations: the call to Mage::getStoreConfig('postcodenl/config/api_secret') returns either null (new installations) or a string (old installations), but the call to trim always results in a string; therefore the !== null comparison always returns true.
The check whether there is an old configuration always returns true, even for new installations: the call to Mage::getStoreConfig('postcodenl/config/api_secret') returns either null (new installations) or a string (old installations), but the call to trim always results in a string; therefore the !== null comparison always returns true.