reduce_frequency: on empty stamps sequence, do not stop

Depending on the policy configuration this condition
can indeed happen in the wild.
1 files changed, 3 insertions(+), 1 deletions(-)

M tshistory_refinery/helper.py
M tshistory_refinery/helper.py +3 -1
@@ 63,7 63,9 @@ def comparator(tsh, engine):
 
 
 def reduce_frequency(tempo, idates):
-    assert len(tempo)
+    if not len(tempo):
+        return tempo
+
     assert len(idates)
     new_tempo = []
     for cdate in tempo: