StreamGroupInfo does not report group lag as null, but 0 when lag is unavailable.

[robhop]

In the special case where consumer group lag is unavailable XINFO GROUPS lag is reported as null.
The Lag property on StreamGroupInfo is long?, so the type is correct.
But when this happens, the value is set to 0 and not null.

Test using Xunit.

 public class LagTest
{
    [Theory]
    [InlineData(false)]
    [InlineData(true)]
    public void GroupLagTest(bool deleteLastEntry)
    {
        // Arrange
        const string stream = "stream";
        const string group = "group";
        NameValueEntry entry = new NameValueEntry("name", "value");

        var db = ConnectionMultiplexer.Connect("localhost").GetDatabase();
        db.Execute("FLUSHDB");
        db.StreamCreateConsumerGroup("stream", "group");
        db.StreamAdd("stream", [entry]);
        db.StreamReadGroup(stream, group, "consumer", "0-0", 1);
        db.StreamAdd("stream", [entry]);

        // Act
        var streamInfo = db.StreamInfo("stream");
        if (deleteLastEntry)
        {
            db.StreamDelete(stream, [streamInfo.LastEntry.Id]);
        }

        // Assert
        var groupInfo = db.StreamGroupInfo("stream").Single(g => g.Name == "group");
        var redisResult = db.Execute("XINFO", "GROUPS", stream);
        long? lag = (long?)redisResult[0][11];

        Assert.Equal(lag, groupInfo.Lag);
    }
}

[mgravell]

fixed by #2902