Fixed issue with TypeConverter<T> where T is Nullable did not w…

…ork.

src/CsvHelper/TypeConversion/TypeConverter.cs

@@ -32,11 +32,6 @@ public abstract class TypeConverter<T> : ITypeConverter
 
 		object ITypeConverter.ConvertFromString(string text, IReaderRow row, MemberMapData memberMapData) => ConvertFromString(text, row, memberMapData);
 
-		string ITypeConverter.ConvertToString(object value, IWriterRow row, MemberMapData memberMapData)
-		{
-			return value is T v
-				? ConvertToString(v, row, memberMapData)
-				: throw new InvalidCastException();
-		}
+		string ITypeConverter.ConvertToString(object value, IWriterRow row, MemberMapData memberMapData) => ConvertToString((T)value, row, memberMapData);
 	}
 }

tests/CsvHelper.Tests/TypeConversion/TypeConverter`1Tests.cs

@@ -0,0 +1,45 @@
+using CsvHelper.Configuration;
+using CsvHelper.TypeConversion;
+using Xunit;
+
+namespace CsvHelper.Tests.TypeConversion
+{
+	public class TypeConverter1Tests
+	{
+		[Fact]
+		public void ConvertToString_NullableBoolean_Converts()
+		{
+			ITypeConverter converter = new GenericBoolConverter();
+			var result = converter.ConvertToString(null, null, null);
+
+			Assert.Equal(string.Empty, result);
+		}
+
+		[Fact]
+		public void ConvertFromString_NullableBoolean_Converts()
+		{
+			ITypeConverter converter = new GenericBoolConverter();
+			var result = (bool?)converter.ConvertFromString("true", null, null);
+
+			Assert.True(result);
+		}
+
+		private class GenericBoolConverter : TypeConverter<bool?>
+		{
+			public override bool? ConvertFromString(string text, IReaderRow row, MemberMapData memberMapData)
+			{
+				if (bool.TryParse(text, out var result))
+				{
+					return result;
+				}
+
+				return null;
+			}
+
+			public override string ConvertToString(bool? value, IWriterRow row, MemberMapData memberMapData)
+			{
+				return value?.ToString() ?? "";
+			}
+		}
+	}
+}