NULL value handling


mysql-test/r/ndb_partition_error.result:
  Fix for bug#15447 Partitions: NULL is treated as zero
    test case
mysql-test/r/partition.result:
  Fix for bug#15447 Partitions: NULL is treated as zero
    test case
mysql-test/t/ndb_partition_error.test:
  Fix for bug#15447 Partitions: NULL is treated as zero
    test case
mysql-test/t/partition.test:
  Fix for bug#15447 Partitions: NULL is treated as zero
    test case
sql/partition_element.h:
  Fix for bug#15447 Partitions: NULL is treated as zero
    added null value flag to partition_element object
sql/partition_info.h:
  Fix for bug#15447 Partitions: NULL is treated as zero
    added null value flag to partition_info object
    added has_null partition id variable