Fix 2 bugs in BoxPlot widget.
Fix 2 bugs in BoxPlot widget.

1) it crashed when it had no input.
2) compare means crashed

kwikadi/orange3,kwikadi/orange3,marinkaz/orange3,marinkaz/orange3,marinkaz/orange3,marinkaz/orange3,marinkaz/orange3,kwikadi/orange3,cheral/orange3,qPCR4vir/orange3,marinkaz/orange3,qPCR4vir/orange3,qusp/orange3,qPCR4vir/orange3,kwikadi/orange3,cheral/orange3,qusp/orange3,qusp/orange3,qPCR4vir/orange3,cheral/orange3,cheral/orange3,kwikadi/orange3,qPCR4vir/orange3,cheral/orange3,kwikadi/orange3,qPCR4vir/orange3,cheral/orange3,qusp/orange3