Prevent Mean of empty slice warning in `fiddy.success.Consistency.m…

…ethod`

Fixes #52.

fiddy/success.py

@@ -1,6 +1,5 @@
 import abc
 from typing import Any, Callable, Union
-
 import numpy as np
 
 from . import analysis, directional_derivative
@@ -98,25 +97,24 @@ def method(
                 equal_nan=self.equal_nan,
             ).all()
 
-        consistent_results = [
+        consistent_results = np.array([
             np.nanmean(list(results_by_size[size].values()), axis=0)
             for size, success in success_by_size.items()
             if success
-        ]
-
-        success = False
-        value = np.nanmean(np.array(consistent_results), axis=0)
-        if consistent_results:
-            success = (
-                np.isclose(
-                    consistent_results,
-                    value,
-                    rtol=self.rtol,
-                    atol=self.atol,
-                    equal_nan=self.equal_nan,
-                ).all()
-                and not np.isnan(consistent_results).all()
-            )
-        value = np.average(np.array(consistent_results), axis=0)
-
+        ])
+
+        if len(consistent_results) == 0:
+            return False, np.nan
+
+        value = np.nanmean(consistent_results, axis=0)
+        success = (
+            np.isclose(
+                consistent_results,
+                value,
+                rtol=self.rtol,
+                atol=self.atol,
+                equal_nan=self.equal_nan,
+            ).all()
+            and not np.isnan(consistent_results).all()
+        )
         return success, value