Add #empty? method to the ChefFS base dir class.

Add #empty? method to the ChefFS base dir (BaseFSDir) class and adjoining spec tests.
Yes. Chef FS file system objects (that represent files) may have the "children" attribute, but if they are at the deepest level within a path, "children" may be empty. For a file, this doesn't indicate that the file is empty, just that there's nothing below it in a path. The same is not true for a directory.