fix assignment import_from_migration external tool rescue in rails 3

setting item.external_tool_tag = nil will trigger destruction
on the old tag

Change-Id: I4b6e257a466a9fceb0a768980019448a2c6a767e
Reviewed-on: https://gerrit.instructure.com/29647
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>