Empty configuration files generate an exception

# Steps to reproduce

1. Create an file in 'releasenotes/config.yaml' containing only the following:

       ---
       # some comment

2. Build docs

# Expected result

Docs build, ignoring the empty configuration file.

# Actual result

The following exception occurs:

  [reno.config] loaded configuration file /tmp/reno-bug-1/releasenotes/config.yaml

  Exception occurred:
      File "/tmp/reno-bug-1/.venv/lib/python2.7/site-packages/reno/config.py", line 224, in _load_file
      self.override(**self._contents)
  TypeError: override() argument after ** must be a mapping, not NoneType