add extra case to _get_unique_job_node_cnt():

when acct_policy_job_runnable_post_select() is call for job without
job_resrcs (eg.: from _attempt_backfill() or when select_nodes() can't
find free recource) but with req_node_bitmap.

This patch will change reason of jobs like in test 21.39 from
"JobState=PENDING Reason=QOSGrpNodeLimit" to
"JobState=PENDING Reason=Resources"

bug 6675