Remove Projects from the sidebar if there are no Projects #977

Similar to the removal of the Projects tab if there are no projects, I think we should remove the Projects section in the issue/PR sidebar if there are no projects to select there.

There are both repository and organisation projects in there so we should check the contents of the selector instead of using the number of projects in the repo as we do for the repository tab.

You cannot create Projects from that selector so there's no reason you would need it unless you already have projects created.