Fix signup for some OAuth providers

some OAuth providers (kerberos for example) only provide a username and an email, but no name. Therefore
a signup fails because the name is empty. Best guess for the name is
probably the username, therefore use it as name.

CHANGELOG

@@ -17,6 +17,7 @@ v 8.4.0 (unreleased)
   - Expire view caches when application settings change (e.g. Gravatar disabled) (Stan Hu)
   - Don't notify users twice if they are both project watchers and subscribers (Stan Hu)
   - Remove gray background from layout in UI
+  - Fix signup for OAuth providers that don't provide a name
   - Implement new UI for group page
   - Implement search inside emoji picker
   - Add API support for looking up a user by username (Stan Hu)

lib/gitlab/o_auth/user.rb

@@ -141,9 +141,12 @@ module Gitlab
           username = auth_hash.username
           email = auth_hash.email
         end
+
+        name = auth_hash.name
+        name = ::Namespace.clean_path(username) if name.strip.empty?
         
         {
-          name:                       auth_hash.name,
+          name:                       name,
           username:                   ::Namespace.clean_path(username),
           email:                      email,
           password:                   auth_hash.password,