Eval: do not join a list if it's empty anyway #2441

This spares some work and also prevents list_join() from calling ga_init()
with a growsize of 0 which would lead to the nvimlog being littered with:

  [warning @ ga_set_growsize:64] 17675 - trying to set an invalid ga_growsize: 0

Also in Vim 7.4.702
  5216f767d4

src/nvim/eval.c

@@ -5418,6 +5418,10 @@ list_join_inner (
  */
 static int list_join(garray_T *gap, list_T *l, char_u *sep, int echo_style, int copyID)
 {
+  if (l->lv_len < 1) {
+    return OK;
+  }
+
   garray_T join_ga;
   int retval;