Fix sacct error for job's not started

This bug was introduced in commit 4a9bffe1
Test12.7 was resulting in the logging of error messages of this sort:
sacct: error: slurmdb_ave_tres_usage: couldn't make tres_list from ''
This was due to the tres_usage_in_ave and tres_usage_out_ave fields
being empty ('\0') if the job's cpu count is zero, which makes
calculation of averages impossible.
bug 2782