FIX: do not suggest "user1" as a username to invited users (#15031)

Recently, the wrong new behavior appeared – we started to suggest to invited users usernames like "user1".

To reproduce:
1. Create an invitation with default settings, do not restrict it to email
2. Copy an invitation link and follow it in incognito mode
See username already filled, with eg “user1”. See screenshot. Should be empty.

This bug was very likely introduced by my recent changes to UserNameSuggester.

app/controllers/invites_controller.rb

@@ -69,11 +69,17 @@ class InvitesController < ApplicationController
 
       hidden_email = email != invite.email
 
+      if hidden_email || invite.email.nil?
+        username = ""
+      else
+        username = UserNameSuggester.suggest(invite.email)
+      end
+
       info = {
         invited_by: UserNameSerializer.new(invite.invited_by, scope: guardian, root: false),
         email: email,
         hidden_email: hidden_email,
-        username: hidden_email ? '' : UserNameSuggester.suggest(invite.email),
+        username: username,
         is_invite_link: invite.is_invite_link?,
         email_verified_by_link: email_verified_by_link
       }