MDL-57383 tool_uploaduser: don't overwrite auth during update.

If updating users during upload along with the option to fill in
missing fields from file content, then we should exclude the auth
column because it will always be present for a user.

admin/tool/uploaduser/classes/process.php

@@ -692,7 +692,11 @@ class process {
             $dologout = false;
 
             if ($this->get_update_type() != UU_UPDATE_NOCHANGES and !$remoteuser) {
-                if (!empty($user->auth) and $user->auth !== $existinguser->auth) {
+
+                // Handle 'auth' column separately, the field can never be missing from a user.
+                if (!empty($user->auth) && ($user->auth !== $existinguser->auth) &&
+                        ($this->get_update_type() != UU_UPDATE_MISSING)) {
+
                     $this->upt->track('auth', s($existinguser->auth).'-->'.s($user->auth), 'info', false);
                     $existinguser->auth = $user->auth;
                     if (!isset($this->supportedauths[$user->auth])) {