piuparts_wrapper: unset scripts dir when non existent

On Ubuntu Precise (Long Term Support release), the piuparts script does
not comes with /etc/piuparts/scripts which causes the piuparts output to
report:

 0m0.0s ERROR: Scripts directory is not a directory:
 /etc/piuparts/scripts/

Add a check to make sure it is a directory, if it is not, fallback to an
empty string passed to piuparts.