Chutney tries to convert empty pid string to integer

In the getPid() function, Chutney reads the pid from a file and converts it to an integer.

if not os.path.exists(pidfile):
    return None

with open(pidfile, 'r') as f:
    return int(f.read())

This can result in the following error:

ValueError: invalid literal for int() with base 10: ''

PR incoming...