sixel: sixel_overwrite_at_cursor(): early exit when the image list is empty

This avoids a call to sixel_overwrite_by_row() (where we also exit
early if the image list is empty).

This saves a couple of instructions to set up the arguments for
sixel_overwrite_by_row().

sixel.c

@@ -610,6 +610,9 @@ sixel_overwrite_by_row(struct terminal *term, int _row, int col, int width)
 void
 sixel_overwrite_at_cursor(struct terminal *term, int width)
 {
+    if (likely(tll_length(term->grid->sixel_images) == 0))
+        return;
+
     sixel_overwrite_by_row(
         term, term->grid->cursor.point.row, term->grid->cursor.point.col, width);
 }