Adsense. Insert members Pub-ID

I’m using the code below to insert authors Adsense Pub-ID from an extra field in their profile. Works great but I would like to insert a default ID if they have not entered one. Any help?

<?php the_author_meta('google_id');?>

<script type="text/javascript"><!--
google_ad_client = "<?php the_author_meta('google_id');?>";
google_ad_width = 468;
google_ad_height = 60;
google_ad_format = "468x60_as";
google_ad_type = "text_image";
google_ad_channel ="";