updated if condition for previous_loss in converged

this for fixing issue qiskit-community/qiskit-aqua#768 that breaks the while loop because of the if condition of the loss value

qiskit/aqua/components/optimizers/aqgd.py

@@ -129,7 +129,7 @@ def converged(self, objval, n=2):
         Returns:
             bool: Whether or not the optimization has converged.
         """
-        if not hasattr(self, '_previous_loss'):
+        if self._previous_loss is None:
             self._previous_loss = [objval + 2 * self._tol] * n
 
         if all([absolute(objval - prev) < self._tol for prev in self._previous_loss]):