`data` parameter for `gio::content_type_guess` should be an `Option`

[According to the documentation](https://developer.gnome.org/gio/stable/gio-GContentType.html#g-content-type-guess), `NULL` is a valid value for the `data` parameter of `g_content_type_guess`. However, the Rust bindings type it as `&[u8]`, making it impossible to pass a null value.