[electrum][trezor] Fix homescreen reset

Summary:
The reset sends a null byte as an image, which is invalid.
Trezor has a feature to restore the default homescreen which is sending a empty bytestream, so let's use it.

Test Plan: Reset the screen on all Trezor devices.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17578