Partition Pruning bug in the case of hive.mapred.mode=nonstrict

When hive.mapred.mode=nonstrict is set, Hive queries doesn't need to have a predicate on partition column. This means the partition expression in the PartitionPruner could be null or it could be null after removing predicates on non-partition columns. We need to handle those cases correctly in terms of whether the resulting partition should be inserted into the "true_parts" or "unknown_parts". The differences is in that if it is true_parts then "select *" queries will be translated to a non_MR job (a fetch task), otherwise it will be translated to a MR job.