Ensure excluded config exists

[klaaspieter]

This ensures we don't attempt to call map on nil when swiftlint is
configured with an empty excluded key.

For example our swiftlint.yml has this:

excluded:
included:
  - folder

Which would make the plugin crash because excluded came in as nil. Since this is valid yml and swiftlint deals with it correctly I think danger-swiftlint should too.

[ashfurrow]

This is great, thanks!