Work around memory-aliasing in descriptor ParsePubkey

Summary:
Having a.size()==0 and then access a[0] is undefined behavior.

Add Span::empty() and use it in script/descriptor

This is a backport of core#19508

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9714