MDL-73826 mod_lti: Fix for Windows/PHP8 with empty curl responses

Sometimes (detected with Windows, when running @ GHA), both the
response and the error of a curl request to non-existing URL
returns the empty string.

In that case, we cannot call to DOMDocument::loadXML() because the
1st param cannot be empty. So here, whenever that happens, we are
throwing the moodle_exception earlier, instead of waiting for the
XML errors to be processed later.

mod/lti/locallib.php

@@ -4392,6 +4392,13 @@ function lti_load_cartridge($url, $map, $propertiesmap = array()) {
     $curl = new curl();
     $response = $curl->get($url);
 
+    // Got a completely empty response (real or error), cannot process this with
+    // DOMDocument::loadXML() because it errors with ValueError. So let's throw
+    // the moodle_exception before waiting to examine the errors later.
+    if (trim($response) === '') {
+        throw new moodle_exception('errorreadingfile', '', '', $url);
+    }
+
     // TODO MDL-46023 Replace this code with a call to the new library.
     $origerrors = libxml_use_internal_errors(true);
     $origentity = lti_libxml_disable_entity_loader(true);