Merge pull request #603 from meziantou/patch-2

Fix nullable annotation of TagName implicit converter

Author: aaubry

YamlDotNet/Core/TagName.cs

@@ -86,6 +86,6 @@ public override int GetHashCode()
             return !(left == right);
         }
 
-        public static implicit operator TagName(string value) => value == null ? Empty : new TagName(value);
+        public static implicit operator TagName(string? value) => value == null ? Empty : new TagName(value);
     }
 }