Change default value of radio button to avoid selecting "FALSE" option

[gadenbuie]

Reported: https://community.rstudio.com/t/avoid-pre-selected-answers-in-learnr-multiple-choice-questions/101950

Previously, when the answer label was "FALSE", that value would be selected because the default value of selected in radioButtons() was FALSE.

```{r KC-1, echo=FALSE}
question("TRUE or FALSE: My R Scripts can look sloppy as long as RStudio is able to run it.",
  answer("TRUE", message = "Not quite. Think about all the *computers* you are creating computer programs for."),
  answer("FALSE", correct = TRUE, message = "That's right. You should strive to make your computer programs should be interpretable by other humans as well."),
  allow_retry = FALSE
)
```

[gadenbuie]

Hmm, it seems like character(0) might be a better default value for selected rstudio/shiny#2266 (comment):

FWIW, I think the way to go is to use character(0) to select none. That's how it works for the update functions updateCheckboxInput and updateSelectInput (and the docs say that's how it works for updateRadioButtons, although # 2688 says that's currently not true).

I'm not sure how far back compatibility goes for selected = character(0) in shiny. Thoughts @schloerke?