Make sure get_cli_args() operates on an array

[josephscott]

The get_cli_args() function in wp-admin/includes/class-wp-importer.php asks for an array from $_SERVER['argv']. However, it is possible for $_SERVER['argv'] to be NULL. In that case sizeof() gets called on a non-countable variable, resulting in a warning like this under PHP 7.2:

Warning: sizeof(): Parameter must be an array or an object that implements Countable in /home/wpcom/public_html/wp-admin/includes/class-wp-importer.php on line 283

By making sure that $args is always an array we can fix that:

    if ( !is_array( $args ) ) {
        $args = array();
    }

[ocean90]

In 42771:

Import: Avoid counting an uncountable type when reading arguments passed to a script.

Props josephscott.
Fixes #42898.