BY THÉRÈSE MARGOLIS     Late last month, the self-proclaimed Sahrawi Arab Democratic Republic (SADR) celebrated it annual national day, a date which marked the 42nd anniversary of its formal establishment- That would not seem at all extraordinary, except for the fact that the SADR is a country that does not exist. Much like poor Philip Nolan in Edward Everett Hale’s fictitious “The Man Without a Country,”