Fixed plotting error when no config (model-free case)

predtuner/approxapp.py

@@ -295,6 +295,8 @@ class ApproxTuner(Generic[T]):
             return conf.test_qos if use_test_qos else conf.qos, conf.speedup
 
         def get_points(confs):
+            if not confs:
+                return np.zeros((2, 0))
             sorted_points = np.array(
                 sorted([qos_speedup(c) for c in confs], key=lambda p: p[0])
             ).T