abis: skip e1_input nesting if empty

With this patch, we don't include e1_input if it's empty

[...]
    timeslot 7
     phys_chan_config TCH/F
     hopping enabled 0
e1_input <----------------- empty, it should not show up.
msc
[...]

openbsc/src/libabis/e1_input_vty.c

@@ -76,6 +76,9 @@ static int e1inp_config_write(struct vty *vty)
 {
 	struct e1inp_line *line;
 
+	if (llist_empty(&e1inp_line_list))
+		return CMD_SUCCESS;
+
 	vty_out(vty, "e1_input%s", VTY_NEWLINE);
 
 	llist_for_each_entry(line, &e1inp_line_list, list) {