[electrum] make get_merkle_root.py work on computers were Electrum ABC was never used

Summary:
When Electrum ABC is started for the first time, it creates its user data directory containing a certs directory where SSL certificates are stored. If this directory does not exist, it will fail to connect to servers that use self-signed certificates.

This diff makes the scripts work even if Electrum ABC was never used on a machine. This should fix the CI failure for the new "Update Electrum ABC checkpoints" CI task.

Depends on D15397

Test Plan:

mv ~/.electrum-abc/certs/  ~/.electrum-abc/certs-bck
python scripts/get_merkle_root.py -vv
ls  ~/.electrum-abc/certs/

This fails before this diff, and now it works.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15398