Fix for error when empty data and aesthetic are used. Fixes #587

Author: wch

R/scales-.r

@@ -55,7 +55,7 @@ scales_train_df <- function(scales, df, drop = FALSE) {
 
 # Map values from a data.frame. Returns data.frame
 scales_map_df <- function(scales, df) {
-  if (empty(df) || length(scales$scales) == 0) return()
+  if (empty(df) || length(scales$scales) == 0) return(df)
 
   mapped <- unlist(lapply(scales$scales, scale_map_df, df = df), recursive = FALSE)