Convert list/tuple value to list of IDs only if it contains records

The Modify Header set the start value from the sale or purchase values but when
it is a list or a tuple, it should also ensure it contains records before
converting to a list of ids.

Closes #13915
(grafted from 35fc6e3d271b4eeb7ef123f58085836fdab77228)

--HG--
branch : 7.0

modules/purchase/purchase.py

@@ -2223,7 +2223,8 @@ class ModifyHeader(Wizard):
                     value = str(value)
                 else:
                     value = value.id
-            elif isinstance(value, (list, tuple)):
+            elif (value and isinstance(value, (list, tuple))
+                    and isinstance(value[0], Model)):
                 value = [r.id for r in value]
             values[fieldname] = value
 

modules/sale/sale.py

@@ -2294,7 +2294,8 @@ class ModifyHeader(Wizard):
                     value = str(value)
                 else:
                     value = value.id
-            elif isinstance(value, (list, tuple)):
+            elif (value and isinstance(value, (list, tuple))
+                    and isinstance(value[0], Model)):
                 value = [r.id for r in value]
             values[fieldname] = value