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Describe the bug
A ValueSet resource expanded by LocalTerminologyService/ValueSetExpander causes NullReferenceException during serialization process (in PocoElementNode ctor due to 'definition' parameter being null).

The issue appears to be the 'expansion' field added by the expander having 'property' field (ValueSet.expansion.contains.property) that exists in R5 or above, not in lower versions.

To Reproduce
Run the code below with Fhir version R4 or below.

    var server = new LocalTerminologyService(
        new MultiResolver(new CachedResolver(ZipSource.CreateValidationSource())));
    var parameters = new Parameters()
    {
        Parameter = new List<Parameters.ParameterComponent>
        {
            new Parameters.ParameterComponent()
            {
                Name = "url",
                Value = new FhirUri("http://hl7.org/fhir/ValueSet/diagnostic-service-sections"),
            },
        },
    };
    var resource = await server.Expand(parameters);
    var s = await resource.ToJsonAsync();

Expected behavior
The json serialization should succeed (the expander should not add any property not existing in a specific Fhir version).

Screenshots
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Version used:

• FHIR Version: R4(B) or below
• Version: 5.11.4

Additional context
Add any other context about the problem here.