For anyone interested this is how I resolved my issue:
if (dtMyTable.Rows[0]["OptionalField"] != null)
{
if (dtMyTable.Rows[0]["OptionalField"].ToString() == "")
{
ddlMyDropDownList .SelectedIndex = 0;
}
else
{
ddlMyDropDownList .SelectedValue = dtMyTable.Rows[0]["OptionalField"].ToString();
}
}