pre_release_branches default to catchall

The option pre_release_branches defaults to ``, but you are using `.match` which always returns true when passed an empty string.

```
branch = "anything";
Boolean(branch.match('')) # always true
```

https://github.com/mathieudutour/github-tag-action/blob/v6.0/src/action.ts#L58

We use this behavior but it is not intuitive (maybe documentation should be updated to say that all branches are prerelease).

Start at src/action.ts#L58 and review how an empty pre_release_branches value is matched. Then find the option's documentation and explain that the default empty value treats all branches as prerelease branches. Done means the documented behavior matches the implementation and the default is clear to users.

Written by the indexing model from the issue text.