Proto: Fix the WASM PVM requests an unknown preimage for invalid hashes

Thomas Letanrequested to merge
lthms@well-known-hash into master

Context

Prior to this patch, the PVM defaults to ask for the preimage of the
zeroed hash, which is unknown as far as we know.

We change its behavior to request the preimage of a well known hash,
so that rollup whose kernel asks for an invalid hash could continue to
progress.

Manually testing the MR

Checklist

  • Document the interface of any function added or modified (see the coding guidelines)
  • Document any change to the user interface, including configuration parameters (see node configuration)
  • Provide automatic testing (see the testing guide).
  • For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
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