[R] remove default values in internal booster manipulation functions

[jameslamb]

Similar to #9457.

Proposes removing default values in internal functions xgb.handleToBooster() and xgb.Booster.handle(), to force calling code to supply all arguments and therefore reduce the risk of "forgot to pass something and silently fell back to a default" types of bugs.

Thanks for your time and consideration.

[trivialfis]

Thank you for the refactoring!

[jameslamb]

@trivialfis there are a few more small changes like this I'd like to propose... would you prefer that I group them into larger PRs? I was doing small PRs to make the reviews easier, but I see now that more activity in the repo can lead to some tests not running

https://buildkite.com/xgboost/xgboost-ci/builds/3177#0189e083-ef62-4968-a00d-7b89e7124063

RuntimeError: Testing rejected. Reason: Today's spending (36.74 USD) exceeded the budget (30.00 USD) allocated for today! The spending limit gets reset every midnight UTC.

I wouldn't want my tiny refactorings to box out testing on more impactful PRs.

[trivialfis]

Thank you for being considerate! Feel free to open PRs based on your own preference. We are heading toward a new release and don't have any major work going.

I think I can review larger cleanup PRs unless there's major optimization/overhaul, feel free to make your own schedule.

[jameslamb]

Ok thanks very much, please do tell me if that ever changes. I want to be respectful of your time and the project's limited resources.