allow empty post-op

nifty4/probing/utils.py

@@ -51,8 +51,10 @@ class StatCalculator(object):
 def probe_with_posterior_samples(op, post_op, nprobes):
     sc = StatCalculator()
     for i in range(nprobes):
-        sample = post_op(op.draw_sample(from_inverse=True))
-        sc.add(sample)
+        if post_op is None:
+            sc.add(op.draw_sample(from_inverse=True))
+        else:
+            sc.add(post_op(op.draw_sample(from_inverse=True)))
 
     if nprobes == 1:
         return sc.mean, None