CSN1: fix serialization with unset value

pycrate_csn1/csnobj.py

@@ -929,6 +929,7 @@ class CSN1List(CSN1Obj):
     
     def _to_pack_obj(self):
         ret = []
+        if self._val is not None:
             for i, val in enumerate(self._val):
                 Obj = self._list[i]
                 # transfer offset and value to Obj