Solana: use block_height for SPECIAL_BLOCK_NUM column and skip empty slots #1608
Description
According to the JSON-RPC spec, block_height is optional. But for slots that resulted in a finalized blocks, it should be present. Our query engine needs the block_hash and previous_block_hash fields to be correct and valid. Currently, for empty slots, they will both be set to empty strings and things will break.
If we find sometime during sync that the block_height is missing for some block, we should look into that block. But this really shouldn't happen.