Why SkipEmptyDataBlocksForward in the end of SeekToFirst

Comment of `Iterator::SeekToFirst` says that the iterator is valid iff the source is not empty, I think if the first element is corrupted, the iterator should be invalid, like `Block::Iter` does.
`TwoLevelIterator::SeekToFirst`'s definition is :
```
index_iter_.SeekToFirst();
InitDataBlock();
if (data_iter_.iter() != nullptr) data_iter_.SeekToFirst();
SkipEmptyDataBlocksForward();
```
Let's say that we use it to iterate over single SST. If the first key-value pair is corrupted, `SkipEmptyDataBlocksForward` will move to the next available data block rather than invalidate the whole iterator.

The `Block::Iter` and `TwoLevelIterator` behaves differently IMO, what's the purpose? Thanks.