Hashset check count before enumerating using Linq

How can i look for count before using linq to enumerate a Hashset? If teh count is zero, i want to return empty
code fiddle

    using System;
using System.Collections.Generic;
using System.Linq;

public class Program
{
    private static HashSet<string> signalsobject = new HashSet<string>();
    public enum Signals : long
    {
        SHORT20EMASIGNAL = 2 ^ 7,
        DOUBLESHORTSIGNAL = 2 ^ 8,
    }

    ;
    public static void Main()
    {
        signalsobject.Add(Signals.SHORT20EMASIGNAL.ToString());
        signalsobject.Add(Signals.SHORT20EMASIGNAL.ToString());
        signalsobject.Remove(Signals.SHORT20EMASIGNAL.ToString());
        Console.WriteLine(signalsobject.Count);
        Console.WriteLine(signalsobject.Aggregate((a, b) => a + " " + b));
    //Console.WriteLine(signalsobject.ToString());
    //  string a = signalsobject.Count>0? (signalsobject.Aggregate((a, b) => a + " " + b)): "Empty";    
    }
}

I currently get
[System.InvalidOperationException: Sequence contains no elements]
at System.Linq.Enumerable.Aggregate[TSource](IEnumerable1 source, Func3 func)
at Program.Main() :line 21

>Solution :

Like this? signalsobject.Aggregate("", (a, b) => a + " " + b)