Views: if fieldvalue (external link) is set, use it, if not, link to node
There is no direct way of using conditionals inside of Views.  However, you might be able to fake it out with something like this:


Order the fields in your view in nid, external link, title order.
In your external link field, set empty field behavior to node/[nid]
Always use it as the link in the last rewrite.


In my just woke up and enjoying espresso before getting to some real work state of mind, this should do what you are asking for, eg, linking to node/nid if there is no external link, but using it if it exists.