There are many reason google will show empty Ads,if it's genuine reason like preliminary checks on your site,etc..,sure your ads will display very soon.
I think there is no way you can force Google Ad,So even if you manage to do that, expect your account to be suspended some time later,because that's way of showing AdSense violates Google's TOS.
You shall not, and shall not authorize or encourage any third party to: ... (ii) edit, modify, filter, truncate or change the order of the information contained in any Ad, Link, Ad Unit, Search Result, or Referral Button, or remove, obscure or minimize any Ad, Link, Ad Unit, Search Result, or Referral Button in any way without authorization from Google.