As I have it, the options are "use defaults" / "always trust" / "never trust" -- so "no value specified" is not an option.
Also, it was not on "use system defaults" but on "always trust".
Upgrading this system to 10.9 (mavericks) creates severe trouble: before, non-working Appstore and iTunes Store was minor nuisance, now I cannot even install Java or do a security update! Very weird.
Revert to 10.7.5 from TimeMachine backup: problem as-is (no appstore, no itunes store, and certificates-complaints starting up itunes).
Clean install from a bootable USB stick: all goes well... Then use TimeMachine backup for transferring my data/apps/licences: appstore Kaput! itunes store Kaput! iBooks Kaput!
Retrieving data ...