Guard missing packed payload in message loader

[brothercorvo]

Summary

• guard against missing or empty packed payloads when loading stored LXMF messages
• return an empty message and log the bad file instead of dereferencing a null pointer
• fixes the T-Deck boot crash triggered while refreshing the conversation list with malformed stored message data

Verification

• reproduced the crash from USB boot logs on the T-Deck
• rebuilt and flashed the patched firmware
• verified boot now reaches UIManager initialized and System Ready - Enjoy! while logging the malformed message path instead of panicking