fix blobbasefee opcode

[yerke26]

Resolves #7059

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[LukaszRozmej]

Hm... why is this a problem? IMO it is fine that this is nullable, tests before Cancun, definitely don't have this set. Maybe the difference should be how this null is handled later? And be sure if the block is post-cancun, to initialize cancun fields to default values, even if not provided.