[WEBSITE] Avoid null assignment when there is no link to convert in the release notes

Summary: Due to set -u in the fetch_documentation.sh, the script fails when there is no relative link in the release notes file because REL_LINKS is set to null. Fix it with double quotes so it's set to an empty string/array as expected.

Test Plan:

./scripts/fetch_documentation.sh

Will fail on 0.25.1 before this patch.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11159