Tx Unindexer cannot proceed if block does not exist in DB

[ceyonur]

I’ve seen this on a state-synced node:
WARN [11-28|12:45:31.391] <C Chain> [github.com/ava-labs/coreth/core/rawdb/chain_iterator.go:104](http://github.com/ava-labs/coreth/core/rawdb/chain_iterator.go:104): Failed to decode block body block=1 error=EOF

This puts the unindex iterator process into a stucked state where it tries to unindex first few blocks when a new block accepted: 1- Say block no: 100 got accepted
2- It checks the tail (which is nil or 0), it tries to unindex the first block (block no 1) and then returns since it cannot be found and skips updating the tail.
3- When block no: 101 got accepted it checks the tail (which is again nil or 0) and repeats the process.

I think we should gracefully handle the nil case here and update the tail. Otherwise tail gets stuck at 0 here (since nextNum is not updated)