Issue 3375: Fix accounting problem in page breaking

Issue 3375: Fix accounting problem in page breaking

The code

\markup \stencil #(ly:make-stencil '() empty-interval empty-interval)
\markup *
\markup *
\markup *
\markup *

distributed itself across 5 pages because page accounting was thrown
off by empty intervals.  While the effects were smaller (and did not
propagate to the next page) with the less radically empty intervals
employed as extents of empty-stencil, they were still present.