WebDAV not working with USE_SESSION_CREDENTIALS

When trying to use a repository using session credentials, WebDAV tries to log in to repository backend with blank password.

When logging in to WebDAV, AuthService::logUser() is called with null password and $bypass_pwd = true in core/classes/sabredav/ajaxplorer/class.AJXP_Sabre_AuthBackendBasic.php:88

```php
AuthService::logUser($this->currentUser, null, true);
```

$pwd parameter is used in AuthService::logUser() to set credentials in core/classes/class.AuthService.php:335

```php
if(ConfService::getCoreConf("SESSION_SET_CREDENTIALS", "auth")) {
	list($authId, $authPwd) = $authDriver->filterCredentials($user_id, $pwd);
	AJXP_Safe::storeCredentials($authId, $authPwd);
}
```

But, since $pwd parameter received is null, session password is set to an empty string, causing an error when using repositories using session credentials.