Warning thrown from XML Loading

It appears that we're passing an empty string if there is no XML file to pass to the PHP-SAML library. This is spilling PHP warnings.

Stack trace:
```
[13-Jul-2018 23:26:38 UTC] PHP Warning: DOMDocument::loadXML(): Empty string supplied as input in /vagrant/content/plugins/wp-simple-saml/vendor/onelogin/php-saml/lib/Saml2/Utils.php on line 88
[13-Jul-2018 23:26:38 UTC] PHP Stack trace:
[13-Jul-2018 23:26:38 UTC] PHP 1. {main}() /vagrant/wp/wp-admin/options-general.php:0
[13-Jul-2018 23:26:38 UTC] PHP 2. do_settings_sections() /vagrant/wp/wp-admin/options-general.php:382
[13-Jul-2018 23:26:38 UTC] PHP 3. do_settings_fields() /vagrant/wp/wp-admin/includes/template.php:1337
[13-Jul-2018 23:26:38 UTC] PHP 4. HumanMade\SimpleSaml\Admin\{closure}() /vagrant/wp/wp-admin/includes/template.php:1378
[13-Jul-2018 23:26:38 UTC] PHP 5. HumanMade\SimpleSaml\instance() /vagrant/content/plugins/wp-simple-saml/inc/admin/namespace.php:268
[13-Jul-2018 23:26:38 UTC] PHP 6. apply_filters() /vagrant/content/plugins/wp-simple-saml/inc/namespace.php:191
[13-Jul-2018 23:26:38 UTC] PHP 7. WP_Hook->apply_filters() /vagrant/wp/wp-includes/plugin.php:203
[13-Jul-2018 23:26:38 UTC] PHP 8. HumanMade\SimpleSaml\Admin\get_config() /vagrant/wp/wp-includes/class-wp-hook.php:286
[13-Jul-2018 23:26:38 UTC] PHP 9. OneLogin_Saml2_IdPMetadataParser::parseXML() /vagrant/content/plugins/wp-simple-saml/inc/admin/namespace.php:57
[13-Jul-2018 23:26:38 UTC] PHP 10. OneLogin_Saml2_Utils::loadXML() /vagrant/content/plugins/wp-simple-saml/vendor/onelogin/php-saml/lib/Saml2/IdPMetadataParser.php:98
[13-Jul-2018 23:26:38 UTC] PHP 11. DOMDocument->loadXML() /vagrant/content/plugins/wp-simple-saml/vendor/onelogin/php-saml/lib/Saml2/Utils.php:88
```