Treat initial <mo> of split and multline (and other related environments) as infix (but remove initial space).  Not sure this exactly matches LaTeX, but is closer than current behavior.  Resolves issue #389.
Make routine to skip initial spaces, and have it also skip empty braces.  Issue #389.
Correct condition for skipping empty braces.
Merge branch 'issue389' into develop.  Issue #389.