Fix KeePassXC Secret Service support

[urrpurr]

When KeePassXC is the org.freedesktop.secrets provider, two bugs made it
unusable (this is the fork Komelia bundles; relates to Komelia#1):

1. Startup crash — ArrayIndexOutOfBoundsException
MessageHandler.send reads parameters[0] when a reply is a D-Bus error.
KeePassXC returns error replies with an empty body (e.g. IsLocked),
so parameters is zero-length and the access throws, killing the app on
launch. Guarded the access. (Mainline swiesend v3.0.0-alpha already has
this guard — this just brings the fork current.)

2. Logins never remembered — read always returns IsLocked
SimpleCollection.getSecret calls unlock(), which only unlocks the
collection. KeePassXC locks items independently: the collection
reports unlocked (so unlock() no-ops) while the item stays locked, so
item.getSecret returns IsLocked and no secret comes back — the user is
re-prompted every launch. Now the item is unlocked explicitly before reading.

With both fixes, KeePassXC works end-to-end: login persists across restarts.
Verified on Arch Linux, KeePassXC 2.7.12, Komelia 0.18.5