fix 400 error on /cosmos/tx/v1beta1/txs/block/ with block with no transactions

[h45hc47]

The current condition offset >= blockTxsLn incorrectly rejects valid requests when offset = 0 and blockTxsLn = 0, so on empty blocks node return error

{
    "code": 3,
    "message": "out of range: cannot paginate 0 txs with offset 0 and limit 100: invalid request: invalid request",
    "details": []
}

Changing to offset > blockTxsLn is save because:

1. Iteration bounds are protected: the actual transaction processing loops have their own bounds checking:
   • Forward pagination: i < blockTxsLn && count != limit
   • Reverse pagination: i > 0 && count != limit
2. Invalid offsets are still rejected: offsets greater than the number of transactions (like offset = 1 when blockTxsLn = 0) will still be properly rejected

Now node return valid response on empty blocks.

Issue: #3239