The test thing aside, I was able to reproduce it immediately.
First, make sure logging is enabled. It isn't enabled (iirc) by default in prod mode. Just switch to dev, and you'll be fine.
Also, if you revert the patch, or move to another version, make sure you clear persistence cache (rm -rf ~ezp5/ezpublish/cache/dev/stash/*). The result of the fixed code is cached, and the code won't run again if you don't clear cache.
What I did, on master (should apply to 5.4 as well):
1. reverted e4314cd49a3e8dd0775a9fca526268a3ec84fbeb
2. created a Link object without an URL
3. Made a REST request to the created content. Example with HTTPie: http -a admin:publish GET http://php55-vm.ezpublish5/api/ezp/v2/content/objects/143 'Accept:application/vnd.ez.api.Content+json'
4. Checked that I had an error in my log: [2014-12-19 11:58:00] app.ERROR: URL with ID '0' not found