Make uploading the assets optional when creating a release draft

Summary:
Since we support both BCHA and BCHN, we stopped uploading the assets to
the github release page. This diffs makes the asset directory optional
when using the github-release.sh script to reflect this behavior.

Test Plan:
Was used to create the 0.22.9 draft:
https://github.com/Bitcoin-ABC/bitcoin-abc/releases/tag/untagged-5d8c9a02a8eb88114b7e

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8674