When a logged-out user views the home page, we currently compute a meaningless "assigned tasks" count which just counts every open task. Don't do this.
Ideally ManiphestTaskQuery should get some tightening up too (that is, withOwners(array(null)) should not select every task), but that might affect other stuff and the performance implications of counting every open task are affecting WMF, so just fix the immediate issue for now.