Ignore empty files

I have run the following command in the [capa](https://github.com/mandiant/capa/) project:
```
~/go/bin/addlicense -check .
```

It complains that some empty files are missing the header such as: [`capa/__init__.py`](https://github.com/mandiant/capa/blob/master/capa/__init__.py)

I think empty files do not need a copyright header and I think that `addlicense` should not add headers (or complain with the `-check` option) to empty files. This could be either the default or supported with an argument.