Fixed PHP warning "Warning: mysqli_close(): Couldn't fetch mysqli"

This line causes warning:
```
Warning: mysqli_close(): Couldn't fetch mysqli in path/vendor/joomla/framework/src/Joomla/Database/Mysqli/MysqliDriver.php on line 87
```
The same condition is used at SqlsrvDriver.php and PostgresqlDriver.php.

Discussed here:
https://github.com/joomla-framework/database/commit/98861e2c941bf171db5d2b1ed7aea9988b2c6a10#commitcomment-7657897