[OMEMO] Empty fingerprint request window

[Neustradamus]

We can receive for a contact, an empty fingerprint request, this problem is not new:

• https://i.ibb.co/wgrzN4N/psi-plus-omemo-empty-fingerprint-request.png
• https://i.ibb.co/nfSbjyG/psi-omemo-empty-fingerprint-request-2020.png

[tehnick]

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