Ignore NO_VAL in SLURMDB_PURGE_* macros.

Signed-off-by: Danny Auble <da@schedmd.com>
Print NONE for slurmdb_purge_string if purge == NO_VAL.  addition to commit
9b00f12cbe5
Fix for bad logic in commit 686cd1177c8147
Merge remote-tracking branch 'origin/slurm-14.03'