Do not raise StopIteration when calling pairwise with empty iterable

Closes #12652
(grafted from bf2f33fe3adf63deae1162ab64f0a1c2cc43eee0)

--HG--
branch : 6.0

modules/sale/sale_reporting.py

@@ -24,7 +24,7 @@ from trytond.i18n import lazy_gettext
 
 def pairwise(iterable):
     a, b = tee(iterable)
-    next(b)
+    next(b, None)
     return zip_longest(a, b)
 
 

modules/stock/stock_reporting_margin.py

@@ -24,7 +24,7 @@ from trytond.transaction import Transaction
 
 def pairwise(iterable):
     a, b = tee(iterable)
-    next(b)
+    next(b, None)
     return zip_longest(a, b)