Provide namespace_id default value

After https://gitlab.com/gitlab-org/gitlab/-/merge_requests/70196 was
merged, and testing on staging we noticed that the namespace_id
parameter was missing. This MR is a bugfix, to provide the
namespace_id parameter when it's not provided.

part of:
https://gitlab.com/gitlab-org/gitlab/-/issues/339654

Changelog: fixed

ee/app/controllers/trials_controller.rb

@@ -35,6 +35,7 @@ class TrialsController < ApplicationController
     if params[:glm_source] == 'about.gitlab.com'
       redirect_to(new_users_sign_up_group_path(url_params.merge(trial_onboarding_flow: true)))
     elsif @namespace = helpers.only_trialable_group_namespace
+      params[:namespace_id] = @namespace.id
       apply_trial_and_redirect
     else
       redirect_to select_trials_url(url_params)

ee/spec/controllers/trials_controller_spec.rb

@@ -127,10 +127,9 @@ RSpec.describe TrialsController do
 
         context 'when the ApplyTrialService is successful' do
           it 'applies a trial to the namespace' do
-            gl_com_params = { gitlab_com_trial: true, sync_to_gl: true }
             apply_trial_params = {
               uid: user.id,
-              trial_user:  ActionController::Parameters.new(post_params).permit(:namespace_id).merge(gl_com_params)
+              trial_user:  ActionController::Parameters.new(post_params).permit(:namespace_id).merge(namespace_id: namespace.id, gitlab_com_trial: true, sync_to_gl: true)
             }
 
             expect_next_instance_of(GitlabSubscriptions::ApplyTrialService) do |service|