Danger fails on PRs with no body

[davdroman]

ERROR: Failed to parse JSON: valueNotFound(Swift.String, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "danger", intValue: nil), CodingKeys(stringValue: "github", intValue: nil), CodingKeys(stringValue: "issue", intValue: nil), CodingKeys(stringValue: "body", intValue: nil)], debugDescription: "Expected String value but found null instead.", underlyingError: nil))
ERROR: Dangerfile eval failed at Dangerfile.swift

Explanation: on PRs with no body set, GitHub most likely returns null for the issue's body instead of an empty string, which Danger cannot parse through Codable probably because the body property is non-optional.

Whilst PRs with no description are not very useful, this tool should be resilient enough to handle such a case.

I'll try and come up with a fix for this and make a PR for it.

[davdroman]

A fix is now pending review @ #237.