Add a New CO Person Role form: default COU selection should be blank

When using the "Add a New CO Person Role" form (e.g. /registry/co_person_roles/add/copersonid:1908/co:2) and when COUs are available in the CO, the default drop-down value for the COU field is currently the first COU in the the list. Make this default value blank so that the COU must be explicitly selected. This will allow the textual changes to the Affiliation and Title fields during COU selection to behave consistently.

To recreate the issue:

1. Click “Add” in the “Role Attributes” section of a CO Person;
2. Notice the labels for affiliation and title compared to the selected COU;
3. Change the selected COU in the dropdown. Notice the change in the Affiliation and Title values;
4. Change back to the first COU on the list, notice that the title and affiliation now reflect the selected COU - different behavior from the same state when first opening the form.