Fix debug_getBadBlocks panic due to no receipt (#272)

The code previously assumed that all non pending transactions passed
to newRPCTransaction would have a receipt, but in fact this is not the
case because recorded bad blocks have no receipts.

The only transaction type that relied on having a receipt is the
CeloDynamicFeeTxV2Type which post cel2 stores the base fee in fee
currency in the receipt which allows for calculating the gas price without
state access.

So we fix the panic by simply allowing post cel2 CeloDynamicFeeTxV2Type
to not have a receipt, and if they don't returning nil for the gas price.

Closes celo-org/celo-blockchain-planning#747

internal/ethapi/api.go

@@ -1610,18 +1610,16 @@ func newRPCTransaction(tx *types.Transaction, blockHash common.Hash, blockNumber
 		} else if isGingerbread && isNativeFeeCurrency {
 			// Post gingerbread mined transaction with a native fee currency, we can compute the effective gas price.
 			result.GasPrice = (*hexutil.Big)(effectiveGasPrice(tx, baseFee))
-		} else if isCel2 && tx.Type() == types.CeloDynamicFeeTxV2Type {
-			// Mined post Cel2 celoDynamicFeeTxV2 transaction, we can get the gas price from the receipt
-			// Assert that we should have a receipt
-			if receipt == nil {
-				panic(fmt.Sprintf("no corresponding receipt provided for celoDynamicFeeTxV2 transaction %s", tx.Hash().Hex()))
-			}
+		} else if isCel2 && tx.Type() == types.CeloDynamicFeeTxV2Type && receipt != nil {
+			// Mined post Cel2 celoDynamicFeeTxV2 transaction with non native fee currency, we get the gas price from
+			// the receipt.
 			result.GasPrice = (*hexutil.Big)(receipt.EffectiveGasPrice)
 		} else {
 			// Otherwise this is either a:
 			// -  pre-gingerbread transaction
 			// -  post-gingerbread native fee currency transaction but no base fee was provided
 			// -  post-gingerbread pre-cel2 transaction with a non-native fee currency
+			// -  post cel2 transaction with a non-native fee currency in a bad block (and therefore without a receipt)
 			//
 			// In these cases we can't calculate the gas price.
 			result.GasPrice = nil