Broken `0` edge case in `WP_CLI\Utils\is_json()`

In the following code, the check will return the wrong result in the case of `$argument` being `0`: https://github.com/wp-cli/wp-cli/blob/6df1833c882152fa01146fffafd7a7ce8150439a/php/utils.php#L1388 

The check should rather be like this:

```php
if ( ! is_string( $argument ) || 0 === strlen( $argument ) ) {
```