[clarity] `var-get` of non existing data var returns `none` instead of throwing

According to the docs, a var-get of a not-yet-existing var should throw an error; instead, it seems to be returning none:

clarity-repl v2.8.0
Enter "::help" for usage hints.
Connected to a transient in-memory database.
>> ::advance_chain_tip 1
1 blocks simulated, new height: 1
>> (define-data-var data int 1)
(define-public (foo (block uint)) (ok (at-block (unwrap-panic (get-block-info? id-header-hash block)) (var-get data))))
→ .ST000000000000000000002AMW42H.contract-0 contract successfully stored. Use (contract-call? ...) for invoking the public functions:
>> ::advance_chain_tip 1
1 blocks simulated, new height: 2
>> (contract-call? .contract-0 foo u1)
(ok 1)
>> (contract-call? .contract-0 foo u0)
(ok none)

Reproduce the behavior in clarity-repl using the documented var-get and at-block example from the issue, comparing a not-yet-existing variable with the existing-variable case. Trace the var-get execution path to determine why the missing value returns none; done means the documented missing-variable behavior is restored and covered by a regression check.