skip index without events

refs: CNVS-13987

Found some index records that cannot find a related audit event.  We
were skipping the index records that could not acquire a relevant key so
we should just skip these as well.

Test Plan:
  - Create a corrupted login event using the examples from g/37605.
  - In cqlsh, delete a record in the auditors table so the index records
    cannot reference back to the event record.
  - Run the data fixup.
  - Should complete successfully.

Change-Id: I475515ab0907901af026146af51e1590a962ee0a
Reviewed-on: https://gerrit.instructure.com/41837
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Nick Cloward <ncloward@instructure.com>