fix: correct check for "no accepted samples" in rejection_sample

Closes #466

Author: vpratz

bayesflow/simulators/simulator.py

@@ -50,12 +50,12 @@ def rejection_sample(
                 # we could cast, but this tends to hide mistakes in the predicate
                 raise RuntimeError(f"Predicate must return a boolean type array. Got dtype={accept.dtype}")
 
-            (accept,) = np.nonzero(accept)
-
             if not np.any(accept):
                 # no samples accepted, skip
                 continue
 
+            (accept,) = np.nonzero(accept)
+
             # apply acceptance mask
             samples = {key: np.take(value, accept, axis=axis) for key, value in samples.items()}