Allow optional GitLab MR description

Author: kvvzr

Sources/Danger/GitLabDSL.swift

@@ -216,7 +216,7 @@ public extension GitLab {
         public let changesCount: String
         public let closedAt: Date?
         public let closedBy: User?
-        public let description: String
+        public let description: String?
         public let diffRefs: DiffRefs
         public let downvotes: Int
         public let firstDeployedToProductionAt: Date?