Handle INFINITE64 initialized stats in slurmd

With the change to use tres_usage_type's the stats coming back from the
extern step are initialized to INFINITE64 instead of 0's.

jobacctinfo_create() in _handle_stat_jobacct() initializes jobacct, but
since the job->task[i]->pid == -1, no stats are gathered and the clean
jobacct is returned.

Related commit: fa3a8ff1

Bug 5554