fix(oauth): if username is empty, fail to login (#4232)

* fix(oauth): if username is empty, fail to login

* fix(oauth): return err when failing to find username

* fix(oauth): disable autofill

api/oauth/oauth.go

@@ -88,6 +88,13 @@ func getUsername(token string, configuration *portainer.OAuthSettings) (string,
 		}
 
 		username := values.Get(configuration.UserIdentifier)
+		if username == "" {
+			return username, &oauth2.RetrieveError{
+				Response: resp,
+				Body:     body,
+			}
+		}
+
 		return username, nil
 	}