This is a regression caused by MDL-48212 (see commit https://github.com/moodle/moodle/commit/9bb71899c0774528f588eaf82fe4ba34bde1511b#diff-968b624e975ad3df52446c206926d60aL778)

The problem is that the new class message does not implement the __isset magic method, this method is required when the class uses the __get magic method for exposing private variables.

If you try to apply the empty function over a private variable function that is not empty, the result of the empty function is going to be "true" if the class does not overload the __isset method.

In order to reproduce this:

• Create a forum with forced subscription
• Post in that forum so the enrolled users receive notifications
• Check in the messages table that the fields contexturl and contexturlname are null for that message