Cannot have >1 author with no email address

We've just run into this in h5py. The empty string seems to be regarded as a valid email address:

```
The email '' is associated in version control with 'A', but is currently assigned to B.Please either add A as an alias to B in '.authors.yml' with:

- name: B
  email: 
  aliases:
    - A

or remove  from B.
```

(names redacted)
