Don't pass NULLs to find_scale()

Closes #2599

Author: lionel-

R/scales-.r

@@ -97,6 +97,7 @@ scales_add_defaults <- function(scales, data, aesthetics, env) {
   if (is.null(new_aesthetics)) return()
 
   datacols <- lapply(aesthetics[new_aesthetics], rlang::eval_tidy, data = data)
+  datacols <- compact(datacols)
 
   for (aes in names(datacols)) {
     scales$add(find_scale(aes, datacols[[aes]], env))