File plugin: Separate config files doesn't allow empty stub env file

[savelandr]

Having a stub environment file when putting environments in environments/<env>.yaml doesn't work if the environment is a stub and therefore the file is empty. This is because YAML.load returns false when parsing nothing. The result looks like:

/usr/local/share/gems/gems/tiller-1.0.0/lib/tiller/data/file.rb:39:in `common': undefined method `key?' for false:FalseClass (NoMethodError)
	from /usr/local/share/gems/gems/tiller-1.0.0/bin/tiller:115:in `block in <module:Tiller>'
	from /usr/local/share/gems/gems/tiller-1.0.0/bin/tiller:110:in `each'
	from /usr/local/share/gems/gems/tiller-1.0.0/bin/tiller:110:in `<module:Tiller>'
	from /usr/local/share/gems/gems/tiller-1.0.0/bin/tiller:35:in `<top (required)>'
	from /usr/local/bin/tiller:23:in `load'
	from /usr/local/bin/tiller:23:in `<main>'