Handle not existing user

[f-meloni]

If the user is not registered on GitHub the JSON we receive has {} in the author field for the commit.
This maybe can be fixed at danger-js level to just return nil in that case maybe, but I didn't see any obvious way? (any idea @orta?)
For the moment I added a patch to the problem

[DangerCI]

	Warnings
⚠️	Sources/Danger/GitHubDSL.swift#L154 - Enum element name should be between 3 and 40 characters long: 'id' (identifier_name)
⚠️	Sources/Danger/GitHubDSL.swift#L170 - Variable name should be between 3 and 40 characters long: 'id' (identifier_name)
⚠️	Sources/Danger/GitHubDSL.swift#L184 - Variable name should be between 3 and 40 characters long: 'id' (identifier_name)
⚠️	Sources/Danger/GitHubDSL.swift#L227 - Enum element name should be between 3 and 40 characters long: 'id' (identifier_name)
⚠️	Sources/Danger/GitHubDSL.swift#L240 - Variable name should be between 3 and 40 characters long: 'id' (identifier_name)
⚠️	Sources/Danger/GitHubDSL.swift#L271 - Enum element name should be between 3 and 40 characters long: 'id' (identifier_name)
⚠️	Sources/Danger/GitHubDSL.swift#L291 - Variable name should be between 3 and 40 characters long: 'id' (identifier_name)
⚠️	Sources/Danger/GitHubDSL.swift#L349 - Enum element name should be between 3 and 40 characters long: 'id' (identifier_name)
⚠️	Sources/Danger/GitHubDSL.swift#L377 - Variable name should be between 3 and 40 characters long: 'id' (identifier_name)
⚠️	Sources/Danger/GitHubDSL.swift#L428 - Variable name should be between 3 and 40 characters long: 'id' (identifier_name)
⚠️	Sources/Danger/GitHubDSL.swift#L452 - Enum element name should be between 3 and 40 characters long: 'id' (identifier_name)
⚠️	Sources/Danger/GitHubDSL.swift#L471 - Variable name should be between 3 and 40 characters long: 'id' (identifier_name)
⚠️	Sources/Danger/GitHubDSL.swift#L505 - File should contain 400 lines or less: currently contains 505 (file_length)

Generated by 🚫 Danger Swift against 9a65335

[f-meloni]

Failing because of danger/danger-js#884