Empty values not available after ACL

[aph]

I have created an ACL which restricts number of items in a drop down dynamic field based on agent role. The values in the drop down list are same as process IDs and are supposed to start a process (configured using generic agent).

For one of the agent roles there is only one item available and it is selected as default. This has the effect that every time an agent logged in with the particular role creates a ticket, it becomes a process ticket, which it should not. The drop down field should provide the possibility to start a process and not make it mandatory.

How do I allow empty values when using ACL?