ensure URL is not empty
see also f4f54fda - adding -f to curl caused URL to be empty if release
was not found (was to be created) - the GH REST API returns 404 with
a valid body without -f but fails with empty. Since `-o pipefail` is not
used, the curl error was not used because last command of pipeline is jq.
The REST API returned 4xx on unknown object but valid JSON body. As
written originally, valid JSON was assumed (result queried on null).
So revert back the original curl calls where acquiring JSON.
It looks like git on GH tags without signatures first and thus the
consequent fetch in create_continuous_release.sh fails because it
fetches signed tag (although it is the same object).
See also:
https://github.com/MartinPulec/UltraGrid/actions/runs/2902280934
+ enable debugging in the script (`-x`)
