…irectory

This addresses gh snakemake#1257. If the rule did not create a logfile, an empty
one would be created in its final destination. If the shadow directive
was used, however, Snakemake would try to move the nonexistent logfile
from the shadow directory to its final location. This caused the
observed failures. This commit has Snakemake create the empty file where
Snakemake would have expected the program to create it (i.e. in the
shadow directory). This enables the unshadow operation that takes place
later to succeed.

…ling; fix existence check