fix(postage): prevent nil pointer panic in TopUpBatch on transaction …

[akrem-chabchoub]

Checklist

• I have read the coding guide.
• My change requires a documentation update, and I have done it.
• I have added tests to cover my changes.
• I have filled out the description and linked the related issues.

Description

Before when running this command you will get panic error, now with the fix:
curl -s -X PATCH "http://localhost:1640/stamps/topup/{BATCH_ID}/2" \ -H "Gas-Price: 10"

{"code":500,"message":"cannot topup batch"}

Config:

minimum-gas-tip-cap: 2
full-node: false
swap-enable: false

Logs:

"time"="2026-01-29 13:39:34.554380" "level"="debug" "logger"="node/transaction" "msg"="estimate gas failed" "sender_address"="0xb25D7c21Fd221719826028D371bEabf74eB061EE" "error"="Transaction execution fails"
"time"="2026-01-29 13:39:34.923401" "level"="debug" "logger"="node/api/patch_stamp_topup" "msg"="topup batch: topup failed" "batch_id"="fe5db63cc90c61509744f952c6fae608c7488ce194227d8d31ca9d7fe502b371" "amount"="10" "error"="topup batch: amount 10: Invalid, intrinsic gas too low"
"time"="2026-01-29 13:39:34.923453" "level"="error" "logger"="node/api/patch_stamp_topup" "msg"="topup batch: topup failed"

Open API Spec Version Changes (if applicable)

Motivation and Context (Optional)

Related Issue (Optional)

Screenshots (if appropriate):

[gacevicljubisa]

Yes, this is obvious panic, if error happens, receipt is nil

[gacevicljubisa]

Also, we should return txHash only in case that there is no error:

txHash = receipt.TxHash

so we should adapt for CreateBatch and DiluteBatch

Also, it would be good if this is covered in unit tests as well.