Special-case no attributes in GetCustomAttributeData

[stephentoub]

It's common to ask for attribute information on members that don't have attributes (which you only discover by asking).

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.Collections.Generic;
using System.Reflection;

[MemoryDiagnoser]
public class Program
{
    static void Main(string[] args) => BenchmarkSwitcher.FromAssemblies(new[] { typeof(Program).Assembly }).Run(args);

    private MethodInfo _mi = typeof(Program).GetMethod("MyMethod");

    public void MyMethod() { }

    [Benchmark]
    public IList<CustomAttributeData> Get() => _mi.GetCustomAttributesData();
}

Method	Toolchain	Mean	Ratio	Allocated
Get	\main\corerun.exe	77.13 ns	1.00	48 B
Get	\pr\corerun.exe	72.08 ns	0.93	-

[jkotas]

Failure is #54125