Refactor post-frontier gas constant's init to use non-zero value as sentinel

Using zero as a sentinel for "not present" could cause issues, because zero is also (obviously) a valid integer. I can foresee a scenario where we accidentally assume `OPCODE_SHL` is present where it was set to zero, mypy wouldn't be able to detect it, and we end up with a zero cost operation. Using `int | None` would force developers to assert at the time of use.

That said, if this is already how the code works, changing it would be out of scope for this PR and this should be addressed in a separate issue.

_Originally posted by @SamWilsn in https://github.com/ethereum/execution-specs/pull/2396#discussion_r3023410536_