CICE 5: No prior restart directory
CICE 5: No prior restart directory

CICE 5's driver was failing because `os.listdir` raised an error if
the prior restart directory was set to None.

We now return an empty list if no directory is present.

(Totally untested, hopefully does not break anything...)

nicjhan/payu,marshallward/payu,aidanheerdegen/payu,marshallward/payu,aidanheerdegen/payu