[electrum] fix transaction_dialog when creating a multisig partially signed transaction

Summary: These Two changes were missed in D18324. The error can be triggered with multisig wallets or using the load transaction from text menu with a partially signed transaction.

Test Plan: with a 2-of-m multisig wallet, create a transaction, export the transaction to hex, load it from another wallet, sign, broadcast

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19009