stats: Fix non-worker stats missing

stats: Fix non-worker stats missing

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.

(cherry picked from commit f17204191d3bb2201e6b6b1c4cf2e7a96148e8cd)