CMFActivity: fix dependency on empty lists

e.g. `activate(after_path=())` should ignore `after_path`

product/CMFActivity/skins/activity/SQLBase_validateMessageList.zsql

@@ -25,18 +25,18 @@ FROM
     <dtml-var table>
 WHERE
     processing_node > -10
-<dtml-if expr="method_id is not None">
+<dtml-if expr="method_id">
     AND method_id IN (
 <dtml-in method_id><dtml-sqlvar sequence-item type="string"><dtml-if sequence-end><dtml-else>,</dtml-if></dtml-in>
     )
 </dtml-if>
 <dtml-if expr="message_uid is not None">AND uid = <dtml-sqlvar message_uid type="int"> </dtml-if>
-<dtml-if expr="path is not None">
+<dtml-if expr="path">
     AND path IN (
 <dtml-in path><dtml-sqlvar sequence-item type="string"><dtml-if sequence-end><dtml-else>,</dtml-if></dtml-in>
     )
 </dtml-if>
-<dtml-if expr="tag is not None">
+<dtml-if expr="tag">
     AND tag IN (
 <dtml-in tag><dtml-sqlvar sequence-item type="string"><dtml-if sequence-end><dtml-else>,</dtml-if></dtml-in>
     )