internal/ethapi: add comment explaining return of nil instead of error (#25097)

Co-authored-by: Felix Lange <fjl@twurst.com>

internal/ethapi/api.go

@@ -1581,6 +1581,8 @@ func (s *PublicTransactionPoolAPI) GetRawTransactionByHash(ctx context.Context,
 func (s *PublicTransactionPoolAPI) GetTransactionReceipt(ctx context.Context, hash common.Hash) (map[string]interface{}, error) {
 	tx, blockHash, blockNumber, index, err := s.b.GetTransaction(ctx, hash)
 	if err != nil {
+		// When the transaction doesn't exist, the RPC method should return JSON null
+		// as per specification.
 		return nil, nil
 	}
 	receipts, err := s.b.GetReceipts(ctx, blockHash)