stats: Fix non-worker stats missing

[awelzel]

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Link to redmine ticket:

Follow up for: https://redmine.openinfosecfoundation.org/issues/6732

Describe changes:

Commit b8b8aa69b49ac0dd222446c28d00a50f9fd7d716 used tm_name of the first StatsRecord of a thread block as key for the "threads" object. However, depending on the type of thread, tm_name can be NULL and would result in no entry being included for that thread at all. This caused non-worker metrics to vanish from the "threads" object in the dump-counters output.

This patch fixes this by remembering the first occurrence of a valid tm_name within the per-thread block and adds another unittest to cover this scenario.

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