fix: automerge should be nullable in simplepullrequest (#27)

Author: JamieMagee

src/Octokit.Webhooks/Models/SimplePullRequest.cs

@@ -108,7 +108,7 @@ public sealed record SimplePullRequest
         public AuthorAssociation AuthorAssociation { get; init; }
 
         [JsonPropertyName("auto_merge")]
-        public bool AutoMerge { get; init; }
+        public bool? AutoMerge { get; init; }
 
         [JsonPropertyName("active_lock_reason")]
         public ActiveLockReason? ActiveLockReason { get; init; }