Do not draw unnecessary spaces

[phoe]

CL-USER> (with-output-to-string (*standard-output*) 
           (draw-cons-tree:draw-tree '(1)))
"[o|/]
 |      
 1      
"

CL-USER> (substitute #\_ #\Space *)
"[o|/]
_|______
_1______
"

The trailing underscores above need not be drawn. (Probably same with the trailing newline.)