Actions on an item to be reviewed in single-approver workflow should not show a button "assign to" if there are not other content approvers.
1) Install Kaleo plugin
2) In Control Panel -> Liferay -> Workflow Configuration, set all things to "Single Approver Workflow" (most importantly blogs, because that's what you're testing)
3) Add a blogs portlet
4) Create a user in the Liferay community and then edit permissions on the blogs portlet to give it write access
5) Create a blog post with that user's account
6) You should see "pending" and the post displayed on the blogs page.
7) Log into your administrator account and click "My workflow tasks"
8) Under "assigned to my roles" you should see a task.
9) There is a "Assign To..." button. Unfortunately, it shouldn't be there. If you click it and click OK, you should get an error (see images).
10) The approval will be assigned to nobody