is_shiny_runtime() is FALSE if knitr is not installed (#553)

Co-authored-by: gadenbuie <gadenbuie@users.noreply.github.com>
Co-authored-by: Carson Sievert <cpsievert1@gmail.com>

Author: gadenbuie

R/sysdata.rda

@@ -44,6 +44,7 @@ is_hosted_app <- function() {
 }
 
 is_shiny_runtime <- function() {
+  if (!is_installed("knitr")) return(FALSE)
   isTRUE(grepl("^shiny", knitr::opts_knit$get("rmarkdown.runtime")))
 }