kerberos-identity: Unbreak handling of fresh caches

commit 4acfcc32 attempted to avoid an error variable getting stomped all over by returning FALSE when encountering the error.

Unfortunately, it's actual legitimate for an error to happen in that path and we should proceed anyway.

That can happen when a credential cache is new and not yet initialized, so it won't have a principal associated with it yet.

This commit changes the problematic code to just pass NULL for the error variable, since we don't need it.