fix: correctly return samples when 'group' is None

src/bob/pad/base/pipelines/abstract_classes.py

@@ -49,5 +49,5 @@ class Database(metaclass=ABCMeta):
             for group in groups:
                 samples.extend(self.predict_samples(group=group))
         else:
-            samples.extend(group=None)
+            samples.extend(self.predict_samples(group=group))
         return samples