BenchmarkDotNet doesn't display `null` parameter correctly

[DoctorKrolic]

BenchmarkDotNet version: 0.13.9

My benchmark code:

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

BenchmarkRunner.Run<NullableEqualsBenchmarks>();

public class NullableEqualsBenchmarks
{
    [Params(1, 0, null)]
    public int? Prop { get; set; }

    [Benchmark]
    public int CurrentLoweringNonDefault()
    {
        int? num = Prop;
        int num2 = 42;
        if ((num.GetValueOrDefault() == num2) & num.HasValue)
        {
            return 1;
        }

        return 0;
    }

    [Benchmark]
    public int ProposedLoweringNonDefault()
    {
        if (Prop.GetValueOrDefault() == 42)
        {
            return 1;
        }

        return 0;
    }

    [Benchmark]
    public int CurrentLoweringDefault()
    {
        int? num = Prop;
        int num2 = 0;
        if ((num.GetValueOrDefault() == num2) & num.HasValue)
        {
            return 1;
        }

        return 0;
    }

    [Benchmark]
    public int ProposedLoweringDefault()
    {
        if (Prop.GetValueOrDefault(1) == 0)
        {
            return 1;
        }

        return 0;
    }
}

Result table after run:

Method	Prop	Mean	Error	StdDev
CurrentLoweringNonDefault	?	0.4134 ns	0.0025 ns	0.0023 ns
ProposedLoweringNonDefault	?	0.2143 ns	0.0095 ns	0.0089 ns
CurrentLoweringDefault	?	0.4140 ns	0.0004 ns	0.0004 ns
ProposedLoweringDefault	?	0.2118 ns	0.0089 ns	0.0078 ns
CurrentLoweringNonDefault	0	0.4051 ns	0.0026 ns	0.0024 ns
ProposedLoweringNonDefault	0	0.2104 ns	0.0070 ns	0.0065 ns
CurrentLoweringDefault	0	0.2134 ns	0.0035 ns	0.0030 ns
ProposedLoweringDefault	0	0.0074 ns	0.0015 ns	0.0012 ns
CurrentLoweringNonDefault	1	0.4160 ns	0.0029 ns	0.0027 ns
ProposedLoweringNonDefault	1	0.2154 ns	0.0063 ns	0.0059 ns
CurrentLoweringDefault	1	0.4141 ns	0.0005 ns	0.0004 ns
ProposedLoweringDefault	1	0.2155 ns	0.0064 ns	0.0060 ns

I would expect null to be displayed in Prop column instead of meaningless ?s

[timcassell]

Should be fixed by #2122