Using Sorl-Thumbnail on url field, not always images

Question

I have a field that I want to use sorl thumbnail on to convert links to thumbnails. I have set it up and when the link is an image, it shows perfectly well.

But, when the link is not an image, eg just having http://www.google.com , sorl goes through and tries to create a thumbnail, produces a non-existant file name and therefore a broken image link on the front end. How can I get it to create a thumbnail if it is an image, otherwise not create a thumbnail at all?

I thought {% empty %} would work, but that seems to only work if there is NO url at all.

My code:

{% thumbnail post.imagelink "600x225" crop="center" as im %}
<a href="" data-toggle="modal" data-target="#myModal-{{ post.id }}"><img class="img-responsive" src="{{ im.url }}" width="{{ im.width }}" height="{{ im.height }}"></a>
  {% empty %}
  no image
{% endthumbnail %}