vty: show asciidoc counters: don't create an empty table for osmo_counters

When creating asciidocs for osmo_counter an empty is not useful.
If there aren't any counter, output a hidden comment
Change-Id: Ie2768100e69dcd7d8d77533688585dd9b43c4a5e
diff --git a/src/vty/stats_vty.c b/src/vty/stats_vty.c
index 6c09c38..faa136d 100644
--- a/src/vty/stats_vty.c
+++ b/src/vty/stats_vty.c
@@ -393,6 +393,13 @@
 
 static void asciidoc_counter_generate(struct vty *vty)
 {
+	if (osmo_counters_count() == 0)
+	{
+		vty_out(vty, "// there are no ungrouped osmo_counters%s",
+			VTY_NEWLINE);
+		return;
+	}
+
 	vty_out(vty, "// ungrouped osmo_counters%s", VTY_NEWLINE);
 	vty_out(vty, ".ungrouped osmo counters%s", VTY_NEWLINE);
 	vty_out(vty, "[options=\"header\"]%s", VTY_NEWLINE);