I'm on Windows, and these are my webserver details:
Apache/2.2.21 (Win32) mod_ssl/2.2.21 - PHP 5.35
I have downloaded the latest Git snapshot and it works well mostly on my server, but when it comes to setting results as NULL, it does not allow you to.
Whether the link contains a hyperlink or not, try setting it to NULL, it will not allow you to - and this applies only to tables with referential integrity, not normal tables where there's no link to another one, that works fine.
This applies from 3.48 onwards, I've continually upgraded and tried to reproduce this.
Attached is the database; import it into your 3.49 or snapshot version and see if you can make any records null, it does not seem to let me.