veyron/lib/flags: make it possible to set an empty string for IPHostPortFlag validation!

Change-Id: I9e5e9a24a27c9bd8cf76d4822b307979ec31453a
diff --git a/lib/flags/listen_test.go b/lib/flags/listen_test.go
index 6c1fc6c..1ec6472 100644
--- a/lib/flags/listen_test.go
+++ b/lib/flags/listen_test.go
@@ -43,7 +43,7 @@
 		want  flags.IPHostPortFlag
 		str   string
 	}{
-		{"", flags.IPHostPortFlag{Port: "0"}, ":0"},
+		{"", flags.IPHostPortFlag{Port: ""}, ""},
 		{":0", flags.IPHostPortFlag{Port: "0"}, ":0"},
 		{":22", flags.IPHostPortFlag{Port: "22"}, ":22"},
 		{"127.0.0.1", flags.IPHostPortFlag{IP: lh, Port: "0"}, "127.0.0.1:0"},