dashboardPage function does not show footer when enable_preloader = FALSE

Hi there, 

The function dashboardPage does not show footer of the app when enable_preloader = FALSE. Check the following code:

`  footer <- if (enable_preloader) {
    shiny::tagAppendAttributes(footer, style = "visibility: hidden;")
  }`

If enable_preloader = FALSE, then this code overwrites a NULL value to footer.