Migration 'if' conditions only support booleans

The condition of migrations' if statements must be of boolean type. Calling another migration tool is not supported, see: https://github.com/phpbb/phpbb/blob/release-3.2.2/phpBB/phpbb/db/migrator.php#L761-L766

This does not work as expected:

['if', [

    ['module.exists', ['acp', false, 'ACP_FOO']],

    ['module.remove', ['acp', false, 'ACP_FOO']],

]],

The only place in phpBB core where no boolean type is provided is this: https://github.com/phpbb/phpbb/blob/release-3.2.2/phpBB/phpbb/db/migration/data/v320/add_help_phpbb.php#L36

The effect is that module.remove is always called, which is not too serious because module.remove internally checks if the module exists before trying to delete it.