Gracefully degrade when no releases are detected

Summary:
Building locally on a fresh checkout fails due to the release page not gracefully
degrading when no releases are detected.

Test Plan:

rm -r abc_md_docs
rm _data/github-releases.json
make serve

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8126