Ensure that empty SAI column indexes do not fail on validation after full-SSTable streaming

patch by Andrés de la Peña; reviewed by Caleb Rackliffe for CASSANDRA-19017

Co-authored-by: Andrés de la Peña <a.penya.garcia@gmail.com>
Co-authored-by: Caleb Rackliffe <calebrackliffe@gmail.com>
6 files changed