fw: inform user when no firmware release found

[szczys]

When no firmware release is available for a device, Golioth will return a zero-length CoAP map (0xa0). When this value is received, show a warning message that "No fw release available", instead of the default "Failed to decode desired map"

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 2cd47dd):

https://golioth-zephyr-sdk-doxygen-dev--pr423-szczys-fw-no-rel-vwntikqq.web.app

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[mniestroj]

Good catch! It is definitely a regression. @szczys What do you think about #424 ? It would basically bring the behavior we have previously when QCBOR was used.

[szczys]

Yes, #424 looks like a better solution!