empty markup causes text overlapping in lilypond >2.17

[Eugene Cormier]

So for years I've been making up theory worksheets with lilypond, and in 
order to get extra vertical spacing between questions, I've used an empty 
markup block like: \markup { }

since my upgrade to lilypond 2.18 I've noticed that when the empty markup 
block is added, instead of putting an empty, invisible, line of text, it now 
in fact freaks out and looses vertical space (like less than default, and 
overlaps with previous elements)

Bug? regression? should it not work that way?

here's a small example
http://lilybin.com/0ntmpy/1