malloc Double free detected when unloading a theme

[theashraf]

   #[test]
    fn test_unload_theme() {
        let player = DotLottiePlayer::new(Config {
            autoplay: true,
            ..create_default_config()
        });

        let theme_id = "test_theme";

        assert!(player.load_dotlottie_data(include_bytes!("assets/test.lottie"), WIDTH, HEIGHT));

        assert!(player.load_theme(theme_id), "Expected theme to load");
        assert!(player.load_theme(""), "Expected theme to unload"); // panic here with "malloc: Double free detected"
    }

[hermet]

@tinyjin, perhaps ThorVG should handle empty string input ("") in addition to nullptr.

[theashraf]

@Hermet

When the user invokes dotLottie_player.load_theme(""), we pass a null pointer to the tvg override function.