How do I upgrade this TypeConverter to use the latest version of AutoMapper?

Question

I'm trying to upgrade some code that I didn't write from AutoMapper 4.0.4 to 7.0.1 and I'm running into an issue. There is a TypeConverter that looks like this:

public class BaseListTypeConverter<TCol1, TCol2, T1, T2> : ITypeConverter<TCol1, TCol2>
    where TCol1 : ICollection<T1>
    where TCol2 : ICollection<T2>
    where T1 : class
    where T2 : class
{
    public TCol2 Convert(ResolutionContext context)
    {
        var sourceList = (TCol1)context.SourceValue;
        TCol2 destinationList = default(TCol2);

        if (context.PropertyMap == null
            || context.Parent == null
            || context.Parent.DestinationValue == null)
            destinationList = (TCol2)context.DestinationValue;
        else
            destinationList = (TCol2)context.PropertyMap.DestinationProperty.GetValue(context.Parent.DestinationValue);
...

But the ITypeConverter and ResolutionContext interfaces have now changed. The ResolutionContext no longer has the SourceValue, DestinationValue, PropertyMap, or Parent properties. I thought that since the new signature of the Covert method has parameters for the source and destination objects that I could just omit the first if statement as in:

public class BaseListTypeConverter<TCol1, TCol2, T1, T2> : ITypeConverter<TCol1, TCol2>
    where TCol1 : ICollection<T1>
    where TCol2 : ICollection<T2>
    where T1 : class
    where T2 : class
{
    public TCol2 Convert(TCol1 sourceList, TCol2 destinationList, ResolutionContext context)
    {
...

But the parameter destinationList is coming in as null so apparently I still need whatever logic that if statement is doing, but how do I rewrite it for AutoMapper 7?