ewmh: Don’t push workspaces with width=0 and height=0 (Thanks Atsutane, badboy)

ewmh: Don’t push workspaces with width=0 and height=0 (Thanks Atsutane, badboy)

Even though i3 cannot know the width/height of some workspaces as
long as they are not initialized (say you used workspace 1 and 3,
but not workspace 2), some applications require this information.
In this case, it was Firefox which intersects the available workareas
(see mozilla/gfx/src/gtk/nsScreenGtk.cpp) and did not position some
windows correctly when being confronted with zero-width/height
workspaces.