v4.0: rpc: Fix short-circuit condition going into big-table (backport of #12068)

[mergify]

Problem

While updating the bigtable fallback logic in #7937, we incorrectly short-circuited the check into bigtable. Currently, we always go into bigtable if until is not specified, even if we've hit the limit of transactions to return.

Summary of changes

Only respect found_until if until is provided.

---

This is an automatic backport of pull request #12068 done by Mergify.

[mergify]

If this PR represents a change to the public RPC API:

1. Make sure it includes a complementary update to rpc-client/ (example)
2. Open a follow-up PR to update the JavaScript client @solana/kit (example)

Thank you for keeping the RPC clients in sync with the server API @mergify[bot].

[steviez]

LGTM as SME

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.9%. Comparing base (024f40a) to head (8955248).

Additional details and impacted files

@@            Coverage Diff            @@
##             v4.0   #12167     +/-   ##
=========================================
- Coverage    82.9%    82.9%   -0.1%     
=========================================
  Files         842      842             
  Lines      317881   317881             
=========================================
- Hits       263793   263750     -43     
- Misses      54088    54131     +43     

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