When a user is rendering the project sidebar and any menu item
is not possible to render, we return a `nil` value.

This introduces an edge case when we want to insert another menu item
before or after that menu, because it was returned as `nil` and,
therefore not introduced in the menu item list.

In this MR, we switched to a Null object pattern, in which we return
a nil menu item instead of just a nil value.