how do i link a field to another field in views
In theory, you should be able to use the link module to create a link field within your content, and use a default value pointing to the newly created content - this may require some customization of the code - it accepts tokens, but I don't know if the node id has been assigned at that point or not.

Assuming it works, one way or another, you would then simply include that field in the view, which should display as a link - if it is used, it would link to the external link, otherwise to the content (as default value).

If that isn't reasonable, you could try to use the same field (without modifications or default value) in the view, and create a NO RESULTS BEHAVIOUR that rewrites it to the content - meaning if there is no value in the field, it would link to the content, or use the value of the field (external link) if present.