juju-deployer handles empty config files ungracefully

If you run juju-deployer with an empty config file (e.g. "juju-deployer -c myconfig.cfg -c secrets.cfg" where secrets.cfg is zero bytes) it dies with a stacktrace:

Traceback (most recent call last):
  File "/usr/bin/juju-deployer", line 9, in <module>
    load_entry_point('juju-deployer==0.9.2', 'console_scripts', 'juju-deployer')()
  File "/usr/lib/python3/dist-packages/deployer/cli.py", line 140, in main
    run()
  File "/usr/lib/python3/dist-packages/deployer/cli.py", line 182, in run
    config = ConfigStack(options.configs or [], options.series)
  File "/usr/lib/python3/dist-packages/deployer/config.py", line 34, in __init__
    self.load()
  File "/usr/lib/python3/dist-packages/deployer/config.py", line 92, in load
    for fp in self._resolve_included():
  File "/usr/lib/python3/dist-packages/deployer/config.py", line 141, in _resolve_included
    [files.extend(self._includes(cf)) for cf in self.config_files]
  File "/usr/lib/python3/dist-packages/deployer/config.py", line 141, in <listcomp>
    [files.extend(self._includes(cf)) for cf in self.config_files]
  File "/usr/lib/python3/dist-packages/deployer/config.py", line 123, in _includes
    d = self._yaml_load(config_file)
  File "/usr/lib/python3/dist-packages/deployer/config.py", line 61, in _yaml_load
    services = yaml_result.get('services')
AttributeError: 'NoneType' object has no attribute 'get'

It really should just skip the empty file (or if it really has to, report an error), not just die.