Quickly fix phpqrcode syntax

Summary: phpqrcode has some old looking php syntax. Fix it quickly since it's one line.

Test Plan:
Before this patch, went to add a TOTP token, saw the error about the undefined variable.
After this patch, successfully added a TOTP token, and used it.

Reviewers: avivey, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Maniphest Tasks: T9300

Differential Revision: https://secure.phabricator.com/D14019

externals/phpqrcode/phpqrcode.php

@@ -2937,7 +2937,7 @@
         //----------------------------------------------------------------------
         public function getCode()
         {
-            $ret;
+            $ret = null;
 
             if($this->count < $this->dataLength) {
                 $row = $this->count % $this->blocks;