fix(rawdb, api): fix rawdb.writeSkippedTransaction & api.GetSkippedTransaction

[0xmountaintop]

1. Purpose or design rationale of this PR

Error

$ curl --location --request POST 'http://localhost:8545' --header 'Content-Type: application/json' --data-raw '{"jsonrpc": "2.0","method": "scroll_getSkippedTransaction","params":["0x734c3a3aeb0a1b993b9b3b8f55200c413251cdd2fd71ff8edd3b544da592e228"],"id": 1}'
{"jsonrpc":"2.0","id":1,"error":{"code":-32000,"message":"fail to Unmarshal traces for skipped tx, hash: 0x734c3a3aeb0a1b993b9b3b8f55200c413251cdd2fd71ff8edd3b544da592e228, err: json: Unmarshal(nil *types.BlockTrace)"}}

Summary:

• no need to marshal if traces is nil
• when doing Unmarshal, must use a nil-pointer for the receiver

You can verify using this PoC:

type RPCTransaction struct {
	Traces *types.BlockTrace `json:"traces,omitempty"`
}

func main() {
	tracesBytes, err := json.Marshal(nil)
	if err != nil {
		panic(err) 
	}

	var rpcTx RPCTransaction
	if len(tracesBytes) != 0 {
		if err := json.Unmarshal(tracesBytes, rpcTx.Traces); err != nil {
			fmt.Printf("fail to Unmarshal traces for skipped tx1, err: %v\n", err)
		}
		traces:= &types.BlockTrace{}
		if err := json.Unmarshal(tracesBytes, traces); err != nil {
			fmt.Printf("fail to Unmarshal traces for skipped tx 2, err: %v\n", err)
		}
	}

	var xxx *types.BlockTrace
	tracesBytes, err = json.Marshal(xxx)
	if err != nil {
		panic(err) 
	}

	if len(tracesBytes) != 0 {
		if err := json.Unmarshal(tracesBytes, rpcTx.Traces); err != nil {
			fmt.Printf("fail to Unmarshal traces for skipped tx 3, err: %v\n", err)
		}
	}
}

Have tested this PR locally.

2. PR title

Your PR title must follow conventional commits (as we are doing squash merge for each PR), so it must start with one of the following types:

• build: Changes that affect the build system or external dependencies (example scopes: yarn, eslint, typescript)
• ci: Changes to our CI configuration files and scripts (example scopes: vercel, github, cypress)
• docs: Documentation-only changes
• feat: A new feature
• fix: A bug fix
• perf: A code change that improves performance
• refactor: A code change that doesn't fix a bug, or add a feature, or improves performance
• style: Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc)
• test: Adding missing tests or correcting existing tests

3. Deployment tag versioning

Has the version in params/version.go been updated?

• This PR doesn't involve a new deployment, git tag, docker image tag, and it doesn't affect traces
• Yes

4. Breaking change label

Does this PR have the breaking-change label?

• This PR is not a breaking change
• Yes