Broken: 85a97449051a (blender2.8 HEAD)
Short description of error
Mesh needs at least a face to show its cage.
Exact steps for others to reproduce the error
The simplest test is a mesh made only of verts and edges with a mirror modifier. The mirror mesh only shows in object mode.
Now if you add a plane to its mesh (so it has vertices and faces), the cage shows correctly.