refactor checkpoint options validation with switch statement

[Muzry]

What type of PR is this?

Other

What does this PR do? Why is it needed?
This PR refactors the checkpoint options validation with a switch statement instead of multiple if conditions.

if blockPath == "" && statePath == "" {
    ...
}
if blockPath != "" && statePath == "" {
    ...
}
if blockPath == "" && statePath != "" { // statePath is guaranteed to be non-empty here
    ...
}

The previous code used multiple if statements to handle the combinations of blockPath and statePath, which made the logic a bit repetitive and not very intuitive.

This change replaces them with a switch statement to enhance readability and clarify the flow.

No functional changes are introduced.

Acknowledgements

• I have read CONTRIBUTING.md.
• I have included a uniquely named changelog fragment file.
• I have added a description to this PR with sufficient context for reviewers to understand this PR.