Catch NULL dn values in 1-n-DBRelationship

ldapalchemy/dbutils.py

@@ -59,7 +59,10 @@ class DBRelationship:
 			return self
 		if self.mapcls is not None:
 			return DBRelationshipSet(obj, self.relattr, self.ldapcls, self.mapcls)
-		return self.ldapcls.query.get(getattr(obj, self.relattr))
+		dn = getattr(obj, self.relattr)
+		if dn is not None:
+			return self.ldapcls.query.get(dn)
+		return None
 
 	def __set__(self, obj, values):
 		if self.mapcls is not None: