Fixes segmentation fault when sending null as name or password.

If i enter the bucket password as null then i get a segmentation fault:
This fix forces the password to be a string.

```php
<?php

$cluster = new CouchbaseCluster('localhost');

$bucketPassword = null;
$bucket = $cluster->openBucket('bucket_name', $bucketPassword);

$bucket->get('key');
```

```
Segmentation fault (core dumped)
```