Fluent assertions: skip checking for default fields in Expectation. (for cases if we want to check only defined non-default fields)

IgnoreNullMembersInExpectation.cs

class IgnoreNullMembersInExpectation : IEquivalencyStep
{
    public bool CanHandle(IEquivalencyValidationContext context, IEquivalencyAssertionOptions config) =>
        context.Expectation is null ||
        context.Expectation is ValueType == default;
    public bool Handle(IEquivalencyValidationContext context, IEquivalencyValidator parent, IEquivalencyAssertionOptions config) => true;
}