aren't empty.  Previously this was technically allowed and worked but was
weird.  The optimization I checked a few hours ago also breaks when empty
matrices are multiplied together so I'm just adding this new requirement.

I also had to fix a part of the LU decomposition because it was doing an
empty matrix multiplication at one point.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403104