adapt check for toolbox
adapt check for toolbox

The behavior of Octave 4 changed and the previous check for length would return 1, even when the toolbox is not present. Now it is additionally checked, whether the returned toobox name is empty.

Works on HG1, HG2, octave 3.8.2 and octave 4.0

egeerardyn/matlab2tikz,matlab2tikz/matlab2tikz,matlab2tikz/matlab2tikz,egeerardyn/matlab2tikz,PeterPablo/matlab2tikz