Docs: Specify `blobhash()` behaviour when blob does not exist

[ToonVanHove]

Add description of blobhash() behaviour when the given blob does not exist. This is currently not specified.

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[r0qs]

Thanks @ToonVanHove, I'm already approving, but please remove the trailing whitespace as reported here: https://app.circleci.com/pipelines/github/ethereum/solidity/34603/workflows/0f99fcfa-1229-432c-9a19-d59a9ac4b4bb/jobs/1568548

[r0qs]

Please, also rebase and squash the commits ;)

[ToonVanHove]

Sorry about that, should be fixed!

[ToonVanHove]

I've added it to both the opcode table and the cheatsheet. Let me know if you'd like any further changes!