Empty value in O2M should remove existing record

The changeset b05bedef260a wrongly skip the removal of existing record in the
group. So it was not possible to remove the last record of the field.

issue5878
review31521002

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

@@ -583,9 +583,9 @@ class O2MField(Field):
     def _set_value(self, record, value, default=False):
         self._set_default_value(record)
         group = record.value[self.name]
-        if not value:
-            return
-        if isinstance(value[0], (int, long)):
+        if value is None:
+            value = []
+        if not value or isinstance(value[0], (int, long)):
             mode = 'list ids'
         else:
             mode = 'list values'