Update Db.php

`Line 178` will never be executed, because `count($this->sql)` returns not less than one, even if the file dump.sql is empty.

Cause:
```php
explode("\n", "");
/*
    returns:
    array(1) {
        [0] => string(0) ""
    }
*/
```
Merge pull request #1481 from diszad/patch-3

Update Db.php