?? Coalesce for empty string?

There isn't a built-in way to do this. You could make your extension method return a string or null, however, which would allow the coalescing operator to work. This would be odd, however, and I personally prefer your current approach.

Since you're already using an extension method, why not just make one that returns the value or a default:

string result = s.SiteNumber.ConvertNullOrEmptyTo("No Number");

C# already lets us substitute values for null with ??. So all we need is an extension that converts an empty string to null, and then we use it like this:

s.SiteNumber.NullIfEmpty() ?? "No Number";

Extension class:

public static class StringExtensions
{
    public static string NullIfEmpty(this string s)
    {
        return string.IsNullOrEmpty(s) ? null : s;
    }
    public static string NullIfWhiteSpace(this string s)
    {
        return string.IsNullOrWhiteSpace(s) ? null : s;
    }
}

I know this is an old question - but I was looking for an answer and none of the above fit my need as well as what I ended up using:

private static string Coalesce(params string[] strings)
{
    return strings.FirstOrDefault(s => !string.IsNullOrEmpty(s));
}

Usage:

string result = Coalesce(s.SiteNumber, s.AltSiteNumber, "No Number");

EDIT: An even more compact way of writing this function would be:

static string Coalesce(params string[] strings) => strings.FirstOrDefault(s => !string.IsNullOrEmpty(s));