Refine fix for #9453

Refine fix for #9453

It is better to introduce a dummy blank dir in TEXINPUTS rather than
appending a blank at the end. Even if I have checked that this is not
a problem with MikTeX, some other engine (maybe texlive, but I cannot
check) could not ignore this space and take it as the name of a dir.
In this case, TEXINPUTS would not end with an empty element and the
standard search path would not be inserted there.