driverevent doesn't update xapian cache

[Harald Sitter]

    m_aptBackend = new QApt::Backend(this);
    if (m_aptBackend->init()) {

if there is no xapian cache (e.g. right after installation) or the cache is sufficiently outdated

QApt::Package *pkg = m_aptBackend->package(key);

will yield nullptr even if there actually is a package, it simply isn't in the xapian cache at the time.

this can lead to incorrectly not showing the notification