Backport of the fix for bug #8143: A date with value 0 is treated as a NULL value


mysql-test/r/delete.result:
  Fix for bug #23412: delete rows with null date field
    - test result
mysql-test/t/delete.test:
  Fix for bug #23412: delete rows with null date field
    - test case
sql/sql_delete.cc:
  Fix for bug #23412: delete rows with null date field
    - during SELECT queries processing we convert 'date[time]_field is null'                   
      conditions into 'date[time]_field = 0000-00-00[ 00:00:00]' for not null                  
      DATE and DATETIME fields. To be consistent, we have to do the same for DELETE            
      queries. So we should call remove_eq_conds() in the mysql_delete() as well.              
      Also it may simplify and speed up DELETE queries execution.