Thymeleaf handling null object in Asterisk - spring

Question

I have a Spring boot App with a Thymeleaf Template in which I define a th:object="${guest}" and I try to access it with the Asterisk syntax like th:value="*{lastName}".
If th:object="${guest} is null it produces an Error: Property or field 'firstName' cannot be found on null. I understand why the error appears. But what is the best way to handle it?
Since I am using th:object="${guest} to prefill values in an form. My current working solution is:
<form th:if="${guest}" th:object="${guest}" ... >
<form th:unless="${guest} ... >
and this is the part I do not understand why can't Thmyleafe workout in the Asterisk that the object is null? But the th:if / th:unless statment gets that th:object is null. There is no case in which accessing an attribute of a null Object is suitable.
And if the object is not there at all it is fine.
Is there a better way with less code?
I also tried:
th:value="*{lastName}?:_" - same error.
th:object="${guest}:?_" - this cant be parsed at all.
Is there a way without the th:if / th:unless. The optimal solution would be to do it in one line so everything stays clean. Can I somehow parse an if in the th:object${}
I could not find something in the documentation or else on the web. Only if Asterisk or ${..} is null
Full Code:
The Fragment:
<form th:fragment="guest_form">
<input type="text" id="firstName" name="firstName" placeholder="Vorname" th:value="*{firstName}" required />
<input type="text" id="lastName" name="lastName" placeholder="Nachname" th:value="*{lastName}" required />
<input type="tel" id="phone" name="phone" placeholder="Telefon Nummer" th:value="*{phone}" />
<input type="email" id="email" name="email" placeholder="Emailadresse" th:value="*{email}" />
<input type="street" id="street" name="street" placeholder="Straße" th:value="*{street}" />
<input type="city" id="city" name="city" placeholder="Stadt" th:value="*{city}" />
<input type="zip" id="zip" name="zip" placeholder="Postleitzahl" th:value="*{zip}" />
<input type="hidden" name="code" th:value="${code}" />
<input th:replace="::csrf_token" />
<input type="submit" id="submitButton" value="Bestätigen" />
</form>
The fragment use:
<form th:include=" fragments/fragments.html::guest_form" th:if="${guest}" th:object="${guest}" action="/register" method="POST"></form>
<form th:include="fragments/fragments.html::guest_form" th:unless="${guest}" action="/register" method="POST"></form>
The function:
#PostMapping("/register")
public String register(#RequestParam String code, Model model, HttpSession session,
RedirectAttributes redirectAttributes, #ModelAttribute("guest") #Valid Guest g,
BindingResult bindingResult) {
model.addAttribute("guest", g);
model.addAttribute("code", code);
// form is not fine
if (bindingResult.hasErrors()) {
return "guest/register";
}
// form is not fine
guestRepo.save(g);
Booking booking = new Booking(g, invRepo.findByCode(code).get());
bookingRepo.save(booking);
session.setAttribute(saBooking, booking);
return "redirect:/ticket";
}
g can be null because you can get redirect to this url - to fill out the form - in this moment g is null. I think I could check if g == null an then Guest g = new Guest(); but this doesnt target my question...