[electrum] fix return type handling of transaction.deserialize in trezor and keepkey plugins
Summary:
This is broken since D14470 (not released yet).
Depends on D14502
Test Plan:
python test_runner.py
Send a transaction with a trezor device
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D14504