vmq_tracer: Do not crash when formatting topics without QoS on Unsubs…

[gmalkas]

The Tracer crashes when formatting the list of topics for a Unsubscribe packet.
When handling a Unsubscribe packet, the topics will not have a QoS, but the formatting
function, which is shared with Subscribe, expects a QoS for every topic.

Proposed Changes

This bugfix simply adds a function clause to handle cases where we only have a topic,
with no QoS and no subscription options. The formatting is kept similar to other cases
for consistency.

Types of Changes

• [ x] Bugfix (non-breaking change which fixes issue #XXXX)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (correction or otherwise)
• Cosmetics (whitespace, styles...)
• DevOps (Build scripts, pipelines...)

Checklist

• I have read the CODE_OF_CONDUCT.md document
• All tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if needed)
• Any dependent changes have been merged and published in related repositories
• I have updated changelog (At the bottom of the release version)
• I have squashed all my commits into one before merging

[ioolkos]

@gmalkas I see, thanks!