wgengine/wgcfg: fix validateEndpoints of empty string

Updates tailscale/corp#1238

wgengine/wgcfg/parser.go

@@ -22,10 +22,14 @@ type ParseError struct {
 }
 
 func (e *ParseError) Error() string {
-	return fmt.Sprintf("%s: ‘%s’", e.why, e.offender)
+	return fmt.Sprintf("%s: %q", e.why, e.offender)
 }
 
 func validateEndpoints(s string) error {
+	if s == "" {
+		// Otherwise strings.Split of the empty string produces [""].
+		return nil
+	}
 	vals := strings.Split(s, ",")
 	for _, val := range vals {
 		_, _, err := parseEndpoint(val)

wgengine/wgcfg/parser_test.go

@@ -53,3 +53,21 @@ func TestParseEndpoint(t *testing.T) {
 		t.Error("Error was expected")
 	}
 }
+
+func TestValidateEndpoints(t *testing.T) {
+	tests := []struct {
+		in   string
+		want error
+	}{
+		{"", nil},
+		{"1.2.3.4:5", nil},
+		{"1.2.3.4:5,6.7.8.9:10", nil},
+		{",", &ParseError{why: "Missing port from endpoint", offender: ""}},
+	}
+	for _, tt := range tests {
+		got := validateEndpoints(tt.in)
+		if !reflect.DeepEqual(got, tt.want) {
+			t.Errorf("%q = %#v (%s); want %#v (%s)", tt.in, got, got, tt.want, tt.want)
+		}
+	}
+}