Merge pull request #602 from meziantou/patch-1

Fix nullable annotations for AnchorName implicit converter

YamlDotNet/Core/AnchorName.cs

@@ -71,6 +71,6 @@ public override int GetHashCode()
             return !(left == right);
         }
 
-        public static implicit operator AnchorName(string value) => value == null ? Empty : new AnchorName(value);
+        public static implicit operator AnchorName(string? value) => value == null ? Empty : new AnchorName(value);
     }
 }