Static assert with no message is a C++17 feature and warns on C++14

Summary: Fix the -Wc++17-extensions warnings introduced in D4731.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix, nakihito, markblundeberg

Reviewed By: #bitcoin_abc, nakihito, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D4753