Make update_fix_d7_block_deltas faster

If you have a site with a lot of users update_fix_d7_block_deltas() can take some time. For us, the update function runs 11,673 time each from menu_update_7002() and system_update_7004(), just over 2/3 of the total update function calls, as reported by drush -v. This is quite a lot, especially since we haven't had configurable blocks for 2 years.

Checking ->where('data IS NOT NULL') is nice, but checking for serialize('block') should save enough time to make up for the slow query. The query being slow isn't too much of a problem, for Drupal.org's live server, it completes in about 3 seconds.