Dont emit \space glyph as empty page number

Dont emit \space glyph as empty page number

Werner L reported in 2012 that LP
emits a space glyph referencing
the century schoolbook font at the
top of every page.

If you use gs (old and current) or
pdfcrop to crop output with multiple
pages and print-page-number = ##f
there is excessive white space at the
top of even pages.

ghostscript should handle that better
but a recent gs bug report
http://bugs.ghostscript.com/show_bug.cgi?id=696445
was closed as "WONTFIX".

There is a workaround within lilypond:

\paper {
    [...]
    oddHeaderMarkup = \markup \null
    evenHeaderMarkup = \markup \null
}

Changing " " to "" cured the problem
and appears to have no negative side
effects. Bounding boxes are affected
by this patch certainly so will have
effects in situations where snippets
are used by other third party
programs.