Fix NullReferenceException when serializing null System.Type properties

[fdcastel]

Problem

Serializing an object with a System.Type property set to null throws a NullReferenceException. This was reported via a failing test in PR #1038.

Reproduction:

var serializer = new SerializerBuilder().Build();
var yaml = serializer.Serialize(new { MyType = (Type)null });
// throws NullReferenceException

Root Cause

SystemTypeConverter.WriteYaml() unconditionally casts value to Type and dereferences it without a null check:

var systemType = (Type)value!;  // NullReferenceException when value is null
emitter.Emit(new Scalar(..., systemType.AssemblyQualifiedName!, ...));

Since System.Type is a reference type (unlike value-type converters such as GuidConverter), it can legitimately be null. The null isn't caught by the normal pipeline because CustomSerializationObjectGraphVisitor.Enter() calls WriteYaml() immediately upon finding the converter, before the null-handling path in FullObjectGraphTraversalStrategy gets a chance to intercept.

Fix

• WriteYaml: Added a null check — emits an empty scalar for null values instead of crashing.
• ReadYaml: Added handling for empty scalars — returns null instead of passing an empty string to Type.GetType() which would throw.

Tests Added

Test	Description
CanSerializeNullType	Serializing a null Type property produces valid YAML
CanSerializeNonNullType	Serializing a non-null Type property still works correctly
NullTypeRoundTrips	Null Type survives serialize → deserialize round-trip
NonNullTypeRoundTrips	Non-null Type survives serialize → deserialize round-trip

All 4 tests pass on net47, net6.0, and net8.0.

Closes #1038