Why not to put check first -

if(rs == null)
{}
else
{
  foreach.....
}

Maybe you will need this. Anyway, in case you often need default behavior for empty lists, you may create next extension method.

void Main()
{
    var elements = Enumerable.Range(0,20);
    elements.ForEachWithElse(x=>Console.WriteLine (x),
                            ()=>Console.WriteLine ("no elements at all"));

    elements.Take(0).ForEachWithElse(x=>Console.WriteLine (x),
                            ()=>Console.WriteLine ("no elements at all"));
}

public static class MyExtensions
{    
    public static void ForEachWithDefault<T>(this IEnumerable<T> values, Action<T> function, Action emptyBehaviour)
    {
        if(values.Any())
            values.ToList().ForEach(function);
        else 
            emptyBehaviour();
    }
}

No it does not have an else operator. You could wrap it in an if else

var rs = from cs in db.CsCustomers
                      from ai in db.ArInvoices
                      where cs.CustomerID == ai.CustomerID &&
                      ai.Kategori != null
                      orderby cs.Unit
                      select new
                      {
                          cs.CustomerID,
                          cs.Unit,
                          cs.Name
                      };
if(rs != null && rs.Any()) //for the .Count you have to add using System.Linq;
     foreach (var r in rs)
     {
          c = new TableCell();
          c.Text = r.CustomerID;
          c.RowSpan = jk;
          tr.Cells.Add(c);

          c = new TableCell();
          c.Text = r.Unit;
          c.RowSpan = jk;
          tr.Cells.Add(c);

          c = new TableCell();
          c.Text = r.Name;
          c.RowSpan = jk;
          tr.Cells.Add(c);
     }
}else{
     //the else part 
}

No, there's no foreach-else loop... If you need that functionality, either use a different type of loop (for-loop or maybe do-while-loop) or test for number of elements first.

Here is an article on how to implement something along the lines.

It's your regular every day foreach on IEnumerable but allows you to specify both a "loop-action" and an "else-action" where the latter is executed after the loop unless you break the loop by returning false from the former. It's a tad clumsier than Python but still useful

What you can do instead is to just check it being null/empty with an if test, and then do something in the else part. However, when doing DB stuff, it's better to return an empty list when there are no result, instead of returning null.