jl_print_task_backtraces does not show some task backtraces

[kpamnany]

#51430 made a significant improvement to the quality of jl_print_task_backtraces -- it fixed #47928. However we occasionally still see:

thread (1)     ---- Task 3 (0x7f2fc40cc4c0)
thread (1)          (sticky: 0, started: 1, state: 0, tid: 2)
thread (1)     ---- End task 3

This gives us no indication as to why there is no backtrace. Looking through the code, there are a number of possible failure cases down the call graph, but none of those output any error message. It would be helpful to know why there is no backtrace for a task.