Fix crash when tracking mtime of 0 files

[kmosher]

Hit an issue today where a non-existent config file was causing;

...
return max(os.path.getmtime(name) for name in self.filenames)
ValueError: max() arg is an empty sequence

[coveralls]

Coverage increased (+0.01%) to 96.88% when pulling 6237465 on kmosher:master into 3994e3b on dnephin:master.

[dnephin]

Nice, thanks!

I've fixed the flake8 test failure on master