fix(rpc): fix nil pointer error in /tx and /tx_search (backport t…

…endermint#3355) (tendermint#3357)

when block is absent
Closes tendermint#3352



---

#### PR checklist

- [ ] ~~Tests written/updated~~
- [x] Changelog entry added in `.changelog` (we use
[unclog](https://github.com/informalsystems/unclog) to manage our
changelog)
- [ ] ~~Updated relevant documentation (`docs/` or `spec/`) and code
comments~~
- [x] Title follows the [Conventional
Commits](https://www.conventionalcommits.org/en/v1.0.0/) spec
<hr>This is an automatic backport of pull request tendermint#3355 done by
[Mergify](https://mergify.com).

---------

Co-authored-by: Anton Kaliaev <[email protected]>

.changelog/unreleased/bug-fixes/3352-nil-pointer-tx-search.md

@@ -0,0 +1,2 @@
+- `[rpc]` Fix nil pointer error in `/tx` and `/tx_search` when block is
+  absent ([\#3352](https://github.com/cometbft/cometbft/issues/3352))

rpc/core/tx.go

@@ -35,7 +35,9 @@ func (env *Environment) Tx(_ *rpctypes.Context, hash []byte, prove bool) (*ctype
 	var proof types.TxProof
 	if prove {
 		block := env.BlockStore.LoadBlock(r.Height)
-		proof = block.Data.Txs.Proof(int(r.Index))
+		if block != nil {
+			proof = block.Data.Txs.Proof(int(r.Index))
+		}
 	}
 
 	return &ctypes.ResultTx{
@@ -114,7 +116,9 @@ func (env *Environment) TxSearch(
 		var proof types.TxProof
 		if prove {
 			block := env.BlockStore.LoadBlock(r.Height)
-			proof = block.Data.Txs.Proof(int(r.Index))
+			if block != nil {
+				proof = block.Data.Txs.Proof(int(r.Index))
+			}
 		}
 
 		apiResults = append(apiResults, &ctypes.ResultTx{