Fix method set_pages_access_level

What does this MR do and why?

!1499 (merged) override pages_access_level which made pages_access_level always return private so the original set_pages_acees_level will skip '=' apart, which cause 'not null violation' when create project_feature

  def set_pages_access_level
    self.pages_access_level ||= if ::Gitlab::Pages.access_control_is_forced?
                                  PRIVATE
                                else
                                  self.project&.public? ? ENABLED : PRIVATE
                                end
  end

See The test part was skipped because I can't reproduce the error in the spec.

Buglink : https://dev-ops.gitlab.cn/gitlab-cn/quality/staging/-/pipelines/173263/failures #2821 (closed)

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