- test result.

Problem: we don't proper handle "" value of the --myisam-recover option.

Fix: turn off myisam recovering if --myisam-recover="" is set.


mysql-test/r/show_check.result:
  Fix for bug #30088: Can't disable myisam-recover by a value of "".
    - test result.
mysql-test/t/show_check-master.opt:
  Fix for bug #30088: Can't disable myisam-recover by a value of "".
    - test case.
mysql-test/t/show_check.test:
  Fix for bug #30088: Can't disable myisam-recover by a value of "".
    - test case.
sql/mysqld.cc:
  Fix for bug #30088: Can't disable myisam-recover by a value of "".
    - turn off myisam recovering if --myisam-recover="" is passed.