pmb.helpers.ui.check_option: fix for UI = none
Don't crash on master_staging_systemd (and in the future the merged systemd branch) if "none" is selected as UI.
Fix for:
ERROR: Could not find aport for package: postmarketos-ui-none
Don't crash on master_staging_systemd (and in the future the merged systemd branch) if "none" is selected as UI.
Fix for:
ERROR: Could not find aport for package: postmarketos-ui-none