[electrum] fix compact_size_nbytes for 0-length scripts

Summary: My previous interpretation of how empty scripts are pushed was wrong. There is a 1-byte compact size (with value 0) even in this case

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14447