Fixes CLTools not setting stdout/stderr if empty
Fixes CLTools not setting stdout/stderr if empty

Returning None instead of a file breaks type safety and will make
pipeline fails.

hjanime/VisTrails,hjanime/VisTrails,celiafish/VisTrails,minesense/VisTrails,VisTrails/VisTrails,hjanime/VisTrails,Nikea/VisTrails,VisTrails/VisTrails,Nikea/VisTrails,minesense/VisTrails,hjanime/VisTrails,minesense/VisTrails,VisTrails/VisTrails,Nikea/VisTrails,celiafish/VisTrails,minesense/VisTrails,celiafish/VisTrails,VisTrails/VisTrails,minesense/VisTrails,celiafish/VisTrails,hjanime/VisTrails,VisTrails/VisTrails,Nikea/VisTrails