dt2.HasValue ? dt2.Value.ToString("yyyy-MM-dd hh:mm:ss") : "n/a";

namespace test
{
  class Program
  {
    static void Main(string[] args)
    {
      int? n = null;
      Console.WriteLine(n==null);
      Console.WriteLine(n.HasValue);
    }
  }
}

.method private hidebysig static void  Main(string[] args) cil managed
{
  .entrypoint
  // Code size       36 (0x24)
  .maxstack  2
  .locals init ([0] valuetype [mscorlib]System.Nullable`1<int32> n)
  IL_0000:  ldloca.s   n
  IL_0002:  initobj    valuetype [mscorlib]System.Nullable`1<int32>
  IL_0008:  ldloca.s   n
  IL_000a:  call       instance bool valuetype [mscorlib]System.Nullable`1<int32>::get_HasValue()
  IL_000f:  ldc.i4.0
  IL_0010:  ceq
  IL_0012:  call       void [mscorlib]System.Console::WriteLine(bool)
  IL_0017:  ldloca.s   n
  IL_0019:  call       instance bool valuetype [mscorlib]System.Nullable`1<int32>::get_HasValue()
  IL_001e:  call       void [mscorlib]System.Console::WriteLine(bool)
  IL_0023:  ret
} // end of method Program::Main

namespace test
{
  class Program
  {
    static void Main(string[] args)
    {
      int? n = null;
      Console.WriteLine(n==null);
      Console.WriteLine(n.HasValue);
    }
  }
}

public static readonly String Empty;