ASP.NET MVC Razor: Ternary
@(string.IsNullOrEmpty(Model.Prop) ? "null" : "'" + Model.Prop + "'")

public class Test {

    public string First { get; set; }

    public string Last { get; set; }
}

public class Test {

    [Display(Name = "First Name")]
    [DisplayFormat(DataFormatString = "'{0}'", NullDisplayText = "'null'")]
    public string First { get; set; }

    [Display(Name = "Last Name")]
    [DisplayFormat(DataFormatString = "'{0}'", NullDisplayText = "'null'")]
    public string Last { get; set; }
}

@Html.DisplayFor(model => model.First)

@Html.DisplayFor(model => model.Last)

[DisplayFormat(DataFormatString = "'{0}'", NullDisplayText = "null")]

public class MyViewModel
{
    [DisplayFormat(NullDisplayText = "null", DataFormatString = "'{0}'"]
    public string MyProperty { get; set; }
}

@model MyViewModel
...
@Html.DisplayFor(x => x.MyProperty)