Make sure dive info is displayed correctly at start

Commit cdae2869d1dd ("Show the datafile name even with no dives") was a
little too aggressive in making sure that we show the correct window title
- we only should call show_dive_info(NULL) if there is indeed no dive in
the dive table - otherwise we display empty dive info at program start.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

main.c

@@ -249,7 +249,8 @@ int main(int argc, char **argv)
 		set_filename(filename, FALSE);
 	}
 	report_dives(imported);
-	show_dive_info(NULL);
+	if (dive_table.nr == 0)
+		show_dive_info(NULL);
 	run_ui();
 	exit_ui();
 	return 0;