Minor improvements to IMS-E enrol - better handling of empty <userid>…

…</userid>

enrol/imsenterprise/enrol.php

@@ -526,8 +526,10 @@ function process_person_tag($tagcontents){
     }
     if(preg_match('{<userid>(.*?)</userid>}is', $tagcontents, $matches)){
         $person->username = trim($matches[1]);
-    }elseif($CFG->enrol_imssourcedidfallback){
+    }
+    if($CFG->enrol_imssourcedidfallback && trim($person->username)==''){
       // This is the point where we can fall back to useing the "sourcedid" if "userid" is not supplied
+	 // NB We don't use an "elseif" because the tag may be supplied-but-empty
         $person->username = $person->idnumber;
     }
     if(preg_match('{<email>(.*?)</email>}is', $tagcontents, $matches)){