EZP-23124: Fix bad block node_id from staging

> Fixes http://jira.ez.no/browse/EZP-23124
> Status: not proud, but ready for review

This fixes a problem in content staging where synced blocks for new content will have their `ezm_block.node_id` set to 0, due to the node not existing yet during publishing. The fix should be harmless since it will only set the field to a new value if the current value is `!= 0`.