MapResolver should return null if we ask for a delisted ticker

Expected Behavior

MapResolver.ResolveMapFile(ticker, date) should return null if we ask for a ticker delisted before date

Actual Behavior

It returns the last MapFile with the ticker

Potential Solution

Check the MapResolver.ResolveMapFile date logic.

Reproducing the Problem

This test should pass, but it fails in current master.

[Test]
public void ResolvesDelistedSymbolAsNull()
{
    var mapResolver = new MapFileResolver(new List<MapFile>
    {
        new MapFile(
            "amz",
            new List<MapFileRow>
            {
                new MapFileRow(new DateTime(1998, 01, 02), "uwz"),
                new MapFileRow(new DateTime(1998, 09, 11), "uwz"),
                new MapFileRow(new DateTime(2004, 12, 31), "amz")
            }
        )
    });
    var date = new DateTime(2007, 01, 03);
    var ticker = "AMZ";
    Assert.IsNull(mapResolver.ResolveMapFile(ticker, date));
}

System Information

Windows

Checklist

[x] I have completely filled out this template
[x] I have confirmed that this issue exists on the current master branch
[x] I have confirmed that this is not a duplicate issue by searching issues
[x] I have provided detailed steps to reproduce the issue

QuantConnect / Lean

MapResolver should return null if we ask for a delisted ticker #4049