Issue 2951: Allow music of nominally zero duration to be typeset.

Issue 2951: Allow music of nominally zero duration to be typeset.

This is important for things like incipits without notes or other
material that produces output without spending time.  In particular,
this is interesting for \score in markup which more often than not
only contains few notational elements and where attempts to just get
basic key/clef/timesignature drawings are sometimes failing in
frustrating manners.