Override field value in view?
The easiest way IMO to do this is to use the views field template. Here is a very detailed and helpful tutorial on DO. You can find the field template's name in the views theming tab under the advanced tab.

In terms of implementation for your use-case, the field's $output in the template would be empty if there is no link, and then you can change $output to be the link to that node (which you should be able to access with $row->nid).