plotcal_spw fails with nspw=1

[abhisek176]

Fixed this by adding the following line:
if len(dirs)==0: dirs=['']

[Jordatious]

Thanks @abhisek176. What are the chances of making a pull request for this? Perhaps @Kitchi has time to review something really quick and easy like that for this script he wrote.

[abhisek176]

Thanks @Jordatious . Done.