fix: remove donation message for unauthorized users (#38188)

Co-authored-by: Oliver Eyton-Williams <ojeytonwilliams@gmail.com>

client/src/client-only-routes/ShowCertification.js

@@ -113,6 +113,7 @@ class ShowCertification extends Component {
     if (
       !isDonationDisplayed &&
       userComplete &&
+      signedInUserName &&
       signedInUserName === username &&
       !isDonating
     ) {