sstable: error out when opening empty ssts

[RaduBerinde]

I ran into a problem with SeekGE on an empty sst sometimes panicking with invalidated blockIter used. A few details in the (private) thread:

https://cockroachlabs.slack.com/archives/CAC6K3SLU/p1706130530093809?thread_ts=1706036242.466479&cid=CAC6K3SLU

We should make sure writing and using empty ssts works, because it's a surprising and subtle failure mode. In some corner cases (see #3406), we might end up writing an empty sst.

[RaduBerinde]

I don't think it makes sense to ever write out empty physical ssts. We should add a check when we open one that it isn't empty, to avoid the panicky failure mode.