Bounding box error

Searches that result in an image are missing a 'snippet' element as there are no text in the image to highlight.

This causes the error.
```
    Notice: Undefined index: snippets in islandora_openseadragon_preprocess_islandora_openseadragon_viewer() (line 138 of /var/www/drupal-7.26/sites/all/modules/islandora_openseadragon/islandora_openseadragon.module).
    Warning: Invalid argument supplied for foreach() in islandora_openseadragon_preprocess_islandora_openseadragon_viewer() (line 138 of /var/www/drupal-7.26/sites/all/modules/islandora_openseadragon/islandora_openseadragon.module).
```