Improved quark tests

Instead of using \tl_if_empty:oTF twice, the new version of
quark tests only requires one emptyness test, at the cost of
slightly more complicated macros.  This speeds up the "true"
branch by 2x but slows down the "false" branch by about 1.3x.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5254 de43f980-851b-0410-b2f7-c40aca1f87e0