YAML.load_file to empty file gives false

When parsing a YAML doc.

Got

```
irb(main):002:0> YAML.load_file File.join("spec","fixtures","empty.yml")
=> false
```

Expected

```
irb(main):002:0> YAML.load_file File.join("spec","fixtures","empty.yml")
=> {}
```

Is this expected or would a patch to return an empty hash be accepted?
