fix null timestamp check in database

examples/datalogger.py

@@ -123,7 +123,7 @@ class DataLogger:
         c.execute(sql, (self.dbid,))
         
         rslt = c.fetchone()
-        if rslt and rslt[0] and rslt[1]:            
+        if rslt and rslt[0] and rslt[1] and rslt[0] != "Null" and rslt[1] != "Null":            
             self.last_collect["ts"] = rslt[0]
             self.last_collect["NextRecNbr"] = rslt[1]
             self.log.debug("Datalogger resuming from recnumber {} last uploaded on {}".format(rslt[1], rslt[0]))