feat(route): allow empty listening port in port specification

Support the ":proxy" format where only the proxy port is specified.
When the listening port part is empty, it defaults to 0 instead of
returning a parse error.

internal/route/types/port.go

@@ -27,7 +27,11 @@ func (p *Port) Parse(v string) (err error) {
 		p.Proxy, err = strconv.Atoi(v)
 	case 2:
 		var err2 error
-		p.Listening, err = strconv.Atoi(parts[0])
+		if parts[0] == "" {
+			p.Listening = 0
+		} else {
+			p.Listening, err = strconv.Atoi(parts[0])
+		}
 		p.Proxy, err2 = strconv.Atoi(parts[1])
 		err = gperr.Join(err, err2)
 	default:

internal/route/types/port_test.go

@@ -10,7 +10,6 @@ var invalidPorts = []string{
 	"",
 	"123:",
 	"0:",
-	":1234",
 	"qwerty",
 	"asdfgh:asdfgh",
 	"1234:asdfgh",
@@ -78,6 +77,14 @@ func TestPortValid(t *testing.T) {
 				Proxy:     5678,
 			},
 		},
+		{
+			name:   "valid_lp_empty",
+			inputs: ":1234",
+			expect: Port{
+				Listening: 0,
+				Proxy:     1234,
+			},
+		},
 		{
 			name:   "valid_p",
 			inputs: "5678",