Fix NULL caller function environment

[gadenbuie]

There was a small bug in #546 that would manifest when running an app via shiny::runApp().

When the following app is run via shiny::runApp()

library(shiny)
library(bslib)

ui <- fluidPage(
  navs_bar(nav("Foo", "foo"))
)

shinyApp(ui, function(...) { })

it will currently generate an error

Error in `rlang::env_name()`:
! `env` must be an environment, not `NULL`.

due to runApp()'s use of eval(). This PR catches the case of the calling function's environment being NULL and assumes that means the function is not being called from within Shiny. With this PR, when you exit the app, the expected deprecation warnings are printed.

You can replace navs_bar() with tabsetPanel() to verify that the deprecation warnings are appropriately muffled in tabsetPanel().

library(shiny)
library(bslib)

ui <- fluidPage(
  # navs_bar(nav("Foo", "foo"))
  tabsetPanel(
    tabPanel("Foo", "foo")
  )
)

shinyApp(ui, function(...) { })

(If testing, be sure to run via shiny::runApp().)