erp5_core: Ignore non-existent transitions in Module_listWorkflowTransitionItemList

instead of crashing.

This can happen in workflow if we have:
- transition_x is declared as possible in state_x
- then transition_x gets deleted
In this case in state_x in manage_properties nothing is visible,
which creates an inconsistent in its data, yet functional generally workflow

Since it is easy to create such cases, just ignore them in Module_listWorkflowTransitionItemList
so that mass change state can work