Make Google AdSense banner in footer conditional

_includes/_footer.html

@@ -1,3 +1,4 @@
+{% if site.owner.google.ad-client and site.owner.google.ad-slot %}
 <div class="google-ads">
   <!-- 320 x 50 ad -->
   <script async src="http://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>
@@ -9,5 +10,6 @@
   (adsbygoogle = window.adsbygoogle || []).push({});
   </script>
 </div><!-- /.google-ads -->
+{% endif %}
 
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