Alter weather block behaviour?
Use the hook_block_view_MODULE_DELTA_alter in your custom module to display text if user has not configured the custom location.you can access the current user object from global $user if you want to perform any operation on it.

/**
 * Implements hook_block_view_MODULE_DELTA_alter().
 */
function demo_wipe_block_view_weather_user_alter(&amp;$data, $block) {
//Check here that whether user has configured or not. 
  if(!is_array($data)) {
    $data['subject'] = t('Blah Blah Blah!');
    $data['content'] = t('Blah Blah Blah! link');
  }
}


P.S. : I have tested it on php 5.5.3

I think you should also specify this as a feature request in the module issue queue to set default whether location(that should be in admin configuration) on user whether block if user has not configured the block yet.