Return empty list when calling O2M.get if empty

Test on required state except to have a value evaluated to False in case of
empty O2M.

issue2382
review258002
(transplanted from 9364d03b9467364498b3f0e97b0fd2e0e85dfa10)

tryton/gui/window/view_form/model/field.py

@@ -511,6 +511,8 @@ class O2MField(CharField):
                     get_readonly=readonly)
                 values.pop(parent_name, None)
                 result.append(('create', values))
+        if not result[0][1]:
+            del result[0]
         if record_removed:
             result.append(('unlink', [x.id for x in record_removed]))
         if record_deleted: