$xwiki.activitystream.searchEvents can return null in some occasion

Steps to reproduce: execute the functional tests of the notifications module.

Then, launch the generated wiki again, and write this page:

{{activity authors="xwiki:XWiki.user2" /}}

For some reason, we see this in the logs:

Left side ($events.size()) of '>' operation has null value at 21:xwiki:Main.WebHome[line 873, column 22]

Apparently, this line returns null for some reason, which is not expected. So the "if" just after actually compares null.size() to a number.