If we have an input parameter (or return value) we need to use (nullable).
However, if it is an (inout) or (out) parameter, (optional) is sufficient.
It looks like (nullable) could be used for everything according to the
Annotation documentation, but (optional) is more specific.
The database is an abstract object implemented by the various TLS
backends, which is used by GTlsConnection to lookup certificates
and keys, as well as verify certificate chains.
Also add GTlsInteraction, which can be used to prompt the user
for a password or PIN (used with the database).
https://bugzilla.gnome.org/show_bug.cgi?id=636572
