fix(api): prevent panic in auth when OAuth is enabled (#3179)

api/http/handler/auth/authenticate.go

@@ -52,7 +52,7 @@ func (handler *Handler) authenticate(w http.ResponseWriter, r *http.Request) *ht
 		return &httperror.HandlerError{http.StatusInternalServerError, "Unable to retrieve a user with the specified username from the database", err}
 	}
 
-	if err == portainer.ErrObjectNotFound && settings.AuthenticationMethod == portainer.AuthenticationInternal {
+	if err == portainer.ErrObjectNotFound && (settings.AuthenticationMethod == portainer.AuthenticationInternal || settings.AuthenticationMethod == portainer.AuthenticationOAuth) {
 		return &httperror.HandlerError{http.StatusUnprocessableEntity, "Invalid credentials", portainer.ErrUnauthorized}
 	}