Recently we deprecated the `all_zeros` functions on `Wtxid` and
`Txid` but for some reason our usage of them is not triggering a lint
warning.
Note please that this changes logic slightly, for example by using an
array of `0xFF` bytes instead of all zeros. Done in an effort to make it
even more obvious that the value is a dummy value and not mix it up with
the all zeros being used for coinbase thing.
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