Add bullet to current text

Hi all, I could use some help with VBA. I'm sure this is probably really simple, but I don't know VBA at all. The below code works, but I need to change it so that it will only add a bullet point if in column D and the cell is NOT empty.

Sub Bullet1()
        Dim r As Range
        With Selection
                 For Each r In Selection
            r.Value = " " & ChrW(&O7) & " " & r.Value
        Next
        End With
End Sub

Any help would be greatly appreciated.