When doing sync for blocks older than
MIN_EPOCHS_FOR_BLOB_SIDECARS_REQUESTS, we skip the blobs by range
request, but we then pass en empty blob sequence to
validation, which then fails.
To fix this: Use an Option[Blobsidecars] to allow expressing the
distinction between "empty blob sequence" and "blobs unavailable". Use
the latter for "old" blocks, and don't attempt to run blob validation.
1cf777c64b