In restore, we run a validation step in checkMissingIntroducedSpans() in which
we decode each introduced span in the backup manifests. This decoding step
naively assumed each introduced span was a table span. Incremental backups of
tenants, however, contain introduced tenant key spans, causing the validation
step to inadvertently fail during a restore of a tenant.

This patch skips this validation step for introduced tenant spans as it
does not apply to introduced tenant spans.

Epic: None

Release note: none