Problem was that the partition containing NULL values
was pruned away, since '2001-01-01' < '2001-02-00' but
TO_DAYS('2001-02-00') is NULL.

Added the NULL partition for RANGE/LIST partitioning on TO_DAYS()
function to be scanned too.

Also fixed a bug that added ALLOW_INVALID_DATES to sql_mode
(SELECT * FROM t WHERE date_col < '1999-99-99' on a RANGE/LIST
partitioned table would add it).

mysql-test/include/partition_date_range.inc:
  Bug#20577: Partitions: use of to_days() function leads to selection failures
  
  Added include file to decrease test code duplication
mysql-test/r/partition_pruning.result:
  Bug#20577: Partitions: use of to_days() function leads to selection failures
  
  Added test results
mysql-test/r/partition_range.result:
  Bug#20577: Partitions: use of to_days() function leads to selection failures
  
  Updated test result.
  This fix adds the partition containing NULL values to
  the list of partitions to be scanned.
mysql-test/t/partition_pruning.test:
  Bug#20577: Partitions: use of to_days() function leads to selection failures
  
  Added test case
sql/item.h:
  Bug#20577: Partitions: use of to_days() function leads to selection failures
  
  Added MONOTONIC_*INCREASE_NOT_NULL values to be used by TO_DAYS.
sql/item_timefunc.cc:
  Bug#20577: Partitions: use of to_days() function leads to selection failures
  
  Calculate the number of days as return value even for invalid dates.
  This is so that pruning can be used even for invalid dates.
sql/opt_range.cc:
  Bug#20577: Partitions: use of to_days() function leads to selection failures
  
  Fixed a bug that added ALLOW_INVALID_DATES to sql_mode
  (SELECT * FROM t WHERE date_col < '1999-99-99' on a RANGE/LIST
  partitioned table would add it).
sql/partition_info.h:
  Bug#20577: Partitions: use of to_days() function leads to selection failures
  
  Resetting ret_null_part when a single partition is to be used, this
  to avoid adding the NULL partition.
sql/sql_partition.cc:
  Bug#20577: Partitions: use of to_days() function leads to selection failures
  
  Always include the NULL partition if RANGE or LIST.
  Use the returned value for the function for pruning, even if
  it is marked as NULL, so that even '2000-00-00' can be
  used for pruning, even if TO_DAYS('2000-00-00') is NULL.
  
  Changed == to >= in get_next_partition_id_list to avoid
  crash if part_iter->part_nums is not correctly setup.