[electrum] misc cleanups of ElectrumWindow.broadcast_transaction

Summary:
Minor code cleanups:

• remove unnecessary variables
• invert some logic to remove nesting (indentatation) by returning early when possible
• use x is not None instead of not x when x is an Optional[something]
• move same-line comments to previous line
• add a few typehints

Test Plan: send a transaction

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14847