fix: Incorrect omemo decryption error for Key Transport Messages

Correctly handle OMEMO stanzas that contain a <header> but no <payload>.
These "Key Transport Messages" (heartbeats) are used by some clients
for session maintenance and establishing trust.

Ref: a2726b6a7 made the <payload> element mandatory in the parser.
Ref: 4d49c2b74 the bug became user-visible.

Fixes: https://github.com/profanity-im/profanity/issues/2129
Signed-off-by: Michael Vetter <jubalh@iodoru.org>

src/xmpp/xmpp.h

@@ -132,6 +132,7 @@ typedef enum {
     OMEMO_ERR_DECRYPT_FAILED,
     OMEMO_ERR_INVALID_JID,
     OMEMO_ERR_MUC_SENDER_NOT_FOUND,
+    OMEMO_ERR_KEY_TRANSPORT,
     OMEMO_ERR_OTHER
 } omemo_error_t;