Fixes #283. If the mkdocs.yml is completely empty there is a traceback
Fix: yaml parse result could be an invalid structure
Add a test case for empty config file
Renaming
Merge remote-tracking branch 'upstream/master' into empty-config
Fix: command line options take precedent over YAML config
Merge pull request #288 from kaiix/empty-config

Fixes #283. If the mkdocs.yml is completely empty there is a traceback