missing email of commit's author creats an error on danger-swift

The error text is:

```
ERROR: Failed to parse JSON: valueNotFound(Swift.String, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "danger", intValue: nil), CodingKeys(stringValue: "git", intValue: nil), CodingKeys(stringValue: "commits", intValue: nil), _JSONKey(stringValue: "Index 0", intValue: 0), CodingKeys(stringValue: "author", intValue: nil), CodingKeys(stringValue: "email", intValue: nil)], debugDescription: "Cannot get unkeyed decoding container -- found null value instead", underlyingError: nil))
ERROR: Dangerfile eval failed at Dangerfile.swift
ERROR: Could not get the results JSON file at /var/folders/1f/7q13rc7s2rq3z17bkrqp5_lw0000gn/T/danger/5DCA2208-519A-4C6C-8691-41D129C8199C/danger-response.json
```