Problem/Motivation
The migration of search.settings configuration (d7_search_settings) assumes that the search_default_module variable is always set. Based on the most recent Drupal 7 codebase, this is false. I guess that it is only set if the admin/config/search/settings form was submitted.
Proposed resolution
I think that since the Drupal 7 default value is node, Drupal core has to provide a default value node_search here.