omemo_message_export_encrypted() should not return early if only its
payload is absent.

omemo_message_prepare_encryption() should be able to accept an
outgoing_message without body, which would become a template of a
KeyTransport message.

Signed-off-by: HardenedVault <[email protected]>

According to:
https://xmpp.org/extensions/xep-0384.html#message-structure-description

Nodes for encrypted symmetric <key/> for a particular recipient should be nested
in a <keys/> node with its "jid" attribute set to the recipient's bare JID, with
an optional "kex" attribute for key exchange usage.

Functions related to these above are modified accordingly.

Signed-off-by: HardenedVault <[email protected]>