Fix guess.rb_decay not to raise an error against bad output

qiskit_experiments/curve_analysis/guess.py

@@ -398,7 +398,9 @@ def rb_decay(
     y = y[valid_inds]
     x = x[valid_inds]
 
-    if len(x) < 2:
+    if len(x) == 0:
+        return 0
+    if len(x) == 1:
         # If number of element is 1, assume y(0) = 1.0 and directly compute alpha.
         a = 1.0 - b
         return ((y[0] - b) / a) ** (1 / x[0])

test/curve_analysis/test_guess.py

@@ -207,3 +207,9 @@ def test_rb_decay(self, a, b, alpha):
         alpha_guess = guess.rb_decay(x, y, b=b)
 
         self.assertAlmostEqual(alpha, alpha_guess, delta=alpha * 0.1)
+
+    def test_rb_decay_with_very_bad_output(self):
+        """Test if rb decay guess does not raise an error even for very bad outputs."""
+        x = np.array([1, 2, 3])
+        y = np.array([0.24, 0.22, 0.23])  # all are below b
+        guess.rb_decay(x=x, y=y, b=0.25)