[JIRAALIGN-2113] "Unassigned backlog" is not taken as an option in Capabilities when PI is set to mandatory

Issue Summary

User cannot select "unassigned backlog" as an option to save a capability when the field is set as mandatory

This behaviour is not consistent with other work items such as Features, and prevent users to save work on Capabilities (choosing "unassigned backlog as the intended selection

Steps to Reproduce

1. In administration> details panel> set single Program Increment as mandatory field for Capability, for a program/portfolio
2. Create or open a capability under the same program
3. Try to save with PI set as "Unassigned backlog"

Repeat same operation for feature and notice the difference

Expected Results

When set to only single PI -selection for capabilities and epics, the system should let the user save Unassigned Backlog as an option. 

Actual Results

The system forces the user to select a nominated PI

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available