improve the empty router

@@ -422,6 +422,9 @@ func (leaf *leafInfo) match(wildcardValues []string) (ok bool, params map[string
 // "/admin/" -> ["admin"]
 // "/admin/users" -> ["admin", "users"]
 func splitPath(key string) []string {
+	if key == "" {
+		return []string{}
+	}
 	elements := strings.Split(key, "/")
 	if elements[0] == "" {
 		elements = elements[1:]

@@ -149,7 +149,11 @@ func TestAddTree2(t *testing.T) {
 }
 
 func TestSplitPath(t *testing.T) {
-	a := splitPath("/")
+	a := splitPath("")
+	if len(a) != 0 {
+		t.Fatal("/ should retrun []")
+	}
+	a = splitPath("/")
 	if len(a) != 0 {
 		t.Fatal("/ should retrun []")
 	}