[electrum] support coins with only x_pubkeys and no pubkeys defined

Summary: The wallet stores them like this. It is easy to compute a pubkey from an xpub + derivation, but let's do it lazily rather than do it in __init__ because it is not cheap and is only really needed when serializing or signing.

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14496