Problem is that DATA_FREE in SHOW TABLE STATUS
is not correct when not using innodb_file_per_table.

The solution is to use I_S.PARTITIONS instead.

This is only a small fix for correcting mean record length and
always return 0 if the table is empty.

sql/ha_partition.cc:
  Bug#36312: InnoDB DATA_FREE BUG?
  
  This is only a small fix for correcting mean record length and
  always return 0 if the table is empty.