[release/10.0.1xx] Allow charset to be unset (#50963)

Author: MiYanni

src/BuiltInTools/dotnet-format/Formatters/CharsetFormatter.cs

@@ -68,7 +68,8 @@ private static byte[] GetEncodedBytes(string text, Encoding encoding)
         private static bool TryGetCharset(AnalyzerConfigOptions analyzerConfigOptions, [NotNullWhen(true)] out Encoding? encoding)
         {
             if (analyzerConfigOptions != null &&
-                analyzerConfigOptions.TryGetValue("charset", out var charsetOption))
+                analyzerConfigOptions.TryGetValue("charset", out var charsetOption) &&
+                charsetOption != "unset")
             {
                 encoding = GetCharset(charsetOption);
                 return true;