Fixed AutoFormatting types that do not have converters (#551)

* Fixed AutoFormatting types that do not have converters

* Addressed warning

src/ExtendedXmlSerializer/ExtensionModel/Xml/AutoMemberFormatExtension.cs

@@ -105,8 +105,8 @@ public IConverter Get(TypeInfo parameter)
 			{
 				var underlying = Nullable.GetUnderlyingType(parameter);
 				var decorate   = underlying != null;
-				var converter  = _previous.Get(underlying ?? parameter);
-				var result     = decorate ? new Converter(converter) : converter;
+				var converter  = (decorate ? _previous.Get(underlying) : null) ?? _previous.Get(parameter);
+				var result     = decorate && converter != null ? new Converter(converter) : converter;
 				return result;
 			}
 		}

test/ExtendedXmlSerializer.Tests.ReportedIssues/Issue550Tests.cs

@@ -0,0 +1,36 @@
+using ExtendedXmlSerializer.Configuration;
+using ExtendedXmlSerializer.Tests.ReportedIssues.Support;
+using FluentAssertions;
+using JetBrains.Annotations;
+using Xunit;
+
+namespace ExtendedXmlSerializer.Tests.ReportedIssues
+{
+	public sealed class Issue550Tests
+	{
+		[Fact]
+		public void Verify()
+		{
+			var configuration = new ConfigurationContainer();
+			configuration.UseAutoFormatting();
+			var instance = new Subject
+			{
+				Struct = new ItemStruct()
+			};
+
+			var serializer   = configuration.Create();
+			serializer.Cycle(instance).Should().BeEquivalentTo(instance);
+
+		}
+
+		class Subject
+		{
+
+			[UsedImplicitly] public ItemStruct? Struct;
+
+		}
+		struct ItemStruct
+		{}
+
+	}
+}