During tests of weird keys [0] it was found out that OpenPGP.js does not
handle keys without valid UIDs well.
There are two issues that this change corrects, first one is adding
filtering of user IDs in `getPrimaryUser` such as only users with self
certifications are considered. Without that change using
`getPrimaryUser` on a key without valid UIDs would throw an exception
(`Cannot read property 'verified' of undefined` [1]).
Second issue is correcting condition whether to throw if no UID was
found. Previous condition checked for `userId` variable, but this is
initialized by default and as such is almost always set. This causes
`key.getPrimaryUser()` (without condition) to throw `Could not find user
that matches that user ID`. The condition has been modified to match the
filter condition (checking if any property of `userId` has been
initialized).
[0]: https://gitlab.com/sequoia-pgp/weird-keys/tree/master/openpgpjs
[1]: 576ed08a54/openpgpjs/results/no-bound-uid.pgp.txt
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