Accounted for rare state when namespace prefix is null.

This is a rare state caused by the combination of several factors and extensions.  Let's hope it doesn't break something else out there. :p

src/ExtendedXmlSerializer/ExtensionModel/Xml/XmlWriter.cs

@@ -122,7 +122,7 @@ string Lookup(string parameter)
 				Add(string.Empty, parameter);
 			}
 
-			var result = parameter == this.Get(string.Empty) ? string.Empty : lookup;
+			var result = parameter == this.Get(string.Empty) || parameter == null ? string.Empty : lookup;
 			return result;
 		}
 

test/ExtendedXmlSerializer.Tests.ReportedIssues/Issue423Tests.cs

@@ -0,0 +1,102 @@
+using ExtendedXmlSerializer.Configuration;
+using ExtendedXmlSerializer.Tests.ReportedIssues.Support;
+using FluentAssertions;
+using JetBrains.Annotations;
+using System.Collections.Generic;
+using System.Linq;
+using Xunit;
+
+namespace ExtendedXmlSerializer.Tests.ReportedIssues
+{
+	public sealed class Issue423Tests
+	{
+		[Fact]
+		public void Verify()
+		{
+			var subject = new Subject {Values = {"Hello", "World!"}};
+			var element = new[] {subject};
+
+			var serializer = new ConfigurationContainer().UseOptimizedNamespaces()
+			                                             .EnableImplicitTyping(typeof(Subject))
+			                                             .Create()
+			                                             .ForTesting();
+			serializer.Cycle(element).Should().BeEquivalentTo(element.Cast<Subject>());
+		}
+
+		[Fact]
+		public void VerifyReported()
+		{
+			var config = new NewOptionSetFieldInfo
+			{
+				Values =
+				{
+					{123, "abc"},
+					{456, "def"}
+				}
+			};
+			var element = new[] {config};
+
+			var serializer = new ConfigurationContainer().UseOptimizedNamespaces()
+			                                             .EnableImplicitTypingFromPublicNested<Issue423Tests>()
+			                                             .EnableParameterizedContentWithPropertyAssignments()
+			                                             .Create()
+			                                             .ForTesting();
+
+			serializer.Cycle(config).Should().BeEquivalentTo(config);
+			serializer.Cycle(element).Should().BeEquivalentTo(element.Cast<NewOptionSetFieldInfo>());
+		}
+
+		public class ExistingOptionSetFieldInfo : NewFieldInfo
+		{
+			public ExistingOptionSetFieldInfo() : base(FieldType.OptionSet)
+			{
+				IsExistsOnSource = true;
+			}
+
+			public bool IsExistsOnSource { get; protected set; }
+
+			public string OptionSetName { get; set; }
+		}
+
+		public enum FieldType
+		{
+			OptionSet
+		}
+
+		public class NewFieldInfo
+		{
+			public NewFieldInfo(FieldType type)
+			{
+				Type = type;
+			}
+
+			public FieldType Type { get; }
+
+			public string Name { get; set; }
+
+			public string Description { get; set; } = null;
+
+			public bool IsRequired { get; set; }
+
+			public string TargetName { get; set; }
+
+			public bool ShouldCreateFieldOnTarget { get; set; }
+		}
+
+		public class NewOptionSetFieldInfo : ExistingOptionSetFieldInfo
+		{
+			public NewOptionSetFieldInfo()
+			{
+				IsExistsOnSource = false;
+			}
+
+			public Dictionary<int, string> Values { get; } = new Dictionary<int, string>();
+		}
+
+		sealed class Subject
+		{
+			[UsedImplicitly]
+			public List<string> Values { get; } = new List<string>();
+		}
+	}
+}