Change the default value for `target`

### Prerequisites

- [X] I have written a descriptive issue title
- [X] I have searched existing issues to ensure the feature has not already been requested

### 🚀 Feature Proposal

This proposal aims to make the API simpler to understand; it doesn't change the actual behaviour, in fact it wants to keep it.

The only difference is that instead of using the special value `any` I’m proposing to just use an _empty_ value (`''`, `undefined`…) to achieve same thing.

### Motivation

- we don't need to create a special value to disable the `target` check, we just leave it empty
- we simplify the validation of `target` so it would only accept valid SemVer identifiers

### Example

N/A