Docs: clarify selectInput() NULL handling with multiple = TRUE (#4390)

Extend the @note in `selectizeInput()` and `varSelectizeInput()` to
explain that deleting all selections with `multiple = TRUE` yields
`NULL`, and that `observeEvent()` / `eventReactive()` ignore `NULL`
by default — point users to `ignoreNULL = FALSE`.

Fixes #3844

Co-authored-by: Alex Bajcz <15620636+BajczA475@users.noreply.github.com>
Co-authored-by: Garrick Aden-Buie <garrick@adenbuie.com>

Author: 3 people

R/input-select.R

@@ -187,6 +187,11 @@ needOptgroup <- function(choices) {
 #'   value when it is a single choice input and the empty string is not in the
 #'   `choices` argument. This is to keep compatibility with
 #'   `selectInput(..., selectize = FALSE)`.
+#'
+#'   When `multiple = TRUE`, deleting all selected options yields an input
+#'   value of `NULL`. Because [observeEvent()] and [eventReactive()] ignore
+#'   `NULL` by default, this transition will not trigger them unless you set
+#'   `ignoreNULL = FALSE`.
 #' @export
 selectizeInput <- function(inputId, ..., options = NULL, width = NULL) {
   selectizeIt(
@@ -411,6 +416,11 @@ varSelectInput <- function(
 #'   value when it is a single choice input and the empty string is not in the
 #'   `choices` argument. This is to keep compatibility with
 #'   `selectInput(..., selectize = FALSE)`.
+#'
+#'   When `multiple = TRUE`, deleting all selected options yields an input
+#'   value of `NULL`. Because [observeEvent()] and [eventReactive()] ignore
+#'   `NULL` by default, this transition will not trigger them unless you set
+#'   `ignoreNULL = FALSE`.
 #' @export
 varSelectizeInput <- function(inputId, ..., options = NULL, width = NULL) {
   selectizeIt(