Fix updateDateRangeInput interaction from #1299 & #1315

[gaborcsardi]

The logic is that NULL values are always dropped from the message, so they have no effect. NA values are kept, and will be null values in JS.

Here are two test apps.

shinyApp(
  ui = fluidPage(
    div(
      dateRangeInput("daterange", "Date range:",
                     start = "2001-01-01",
                     end   = "2010-12-31")
    ),
    actionButton("btn1", "leave start & end alone"),
    actionButton("btn11", "reset start & end with NA"),
    actionButton("btn2", "set date range start"),
    actionButton("btn3", "set date range end"),
    actionButton("btn4", "set date range start and end")
  ),
  server = function(input, output, session) {
    observeEvent(input$btn1, updateDateRangeInput(session, "daterange"))
    observeEvent(input$btn11, updateDateRangeInput(session, "daterange", start = NA, end = NA))
    observeEvent(input$btn2, updateDateRangeInput(session, "daterange", start = "2005-01-01"))    
    observeEvent(input$btn3, updateDateRangeInput(session, "daterange", end = "2006-01-01"))
    observeEvent(input$btn4, updateDateRangeInput(session, "daterange", start = "2007-01-01", end = "2008-01-01"))
  }
)

shinyApp(
  ui = fluidPage(
    div(
      dateInput("date", "Date", min = "2014-06-10"),
      dateRangeInput("daterange", "Date range:",
               start = "2001-01-01",
               end   = "2010-12-31")
    ),
    actionButton("btn1", "reset date NA"),
    actionButton("btn1", "reset date NULL"),
    actionButton("btn2", "reset date range start"),
    actionButton("btn3", "reset date range end"),
    actionButton("btn4", "reset date range start and end")
  ),
  server = function(input, output, session) {
    observeEvent(input$btn1, updateDateInput(session, "date", value = NA))
    observeEvent(input$btn1, updateDateInput(session, "date", value = NULL))
    observeEvent(input$btn2, updateDateRangeInput(session, "daterange", start = NA, end = "2005-01-01"))
    observeEvent(input$btn3, updateDateRangeInput(session, "daterange", start = "2003-01-01", end = NA))
    observeEvent(input$btn4, updateDateRangeInput(session, "daterange", start = NA, end = NA))
  }
)