Do not render empty column blocks in the editor when viewing mobile and stacked

[richtabor]

I propose that an empty columns block should not render when columns are set to stack on mobile. On the front-end these do not take up space, nor distort the layout. Completing this would mean the front-end and editor instances are the same, meeting expectations.

One note is that the empty column should be visible when the parent Columns block is selected. This way you can put something in the empty column, but it's not always distorting layout.

Editor, desktop:

Editor, mobile (bug visually distorting layout):

Frontend, mobile (no empty column taking up space):