Last "Next Incident" button redirects to a null incident

[pdcp1]

As part of the testing of #2689 I found an edge-case test.

Steps to reproduce

1. Go to the last incident (at this moment the last incident is https://incidentdatabase.ai/cite/652 )
2. Scroll down and check the Next Incident button. It should be disabled
3. Click that Next Incident button anyways
4. It redirects to https://incidentdatabase.ai/cite/null/

The same happens with the first incident https://incidentdatabase.ai/cite/1/ and the Previous Incident button.

Notes

In addition to the disabled visual state, the button shouldn't be clickable (completely disabled)

[kepae]

I believe this is now fixed.