added early return

by specifying `columns=NULL`, one can programmatically skip this step
diff --git a/R/remove_column.R b/R/remove_column.R
index 3847d95..828edae 100644
--- a/R/remove_column.R
+++ b/R/remove_column.R
@@ -10,6 +10,7 @@
 #' kable() %>% 
 #'     remove_column(2:3)
 remove_column <- function (kable_input, columns) {
+    if(is.null(columns)) return(kable_input)
     kable_format <- attr(kable_input, "format")
     if (!kable_format %in% c("html", "latex")) {
         warning("Please specify format in kable. kableExtra can customize either ",