Support std::optional in FromJSVal

This commit introduces support for std::optional<T> in
Script::FromJSVal. When a JavaScript value is undefined or null, the
resulting optional is set to std::nullopt; otherwise, the value is
converted and wrapped.

This change allows components to cleanly handle optional script values
without needing manual null checks or exception handling in C++.

As a direct application of this feature, the Identity component now uses
std::optional<std::wstring> for the return value of GetCiv(). This
resolves a bug where formation templates (e.g., those inheriting from
template_formation.xml) do not explicitly define a civ. After commit
03f7903fec, the code assumed GetCiv() would always return a valid
string, leading to undefined behavior when it was missing.

With this update:
- GetCiv() returning undefined results in an empty optional.
- The Identity component defaults to an empty civilization string ("")
  when the civ is not defined.
- This avoids crashes or actor parsing errors for civ-less templates and
  improves robustness in script-C++ interaction.

Closes: #8107
Fixes: #8091

source/scriptinterface/ScriptConversions.h

@@ -23,6 +23,8 @@
 #include "ScriptExtraHeaders.h" // for typed arrays
 
 #include <limits>
+#include <optional>
+#include <type_traits>
 #include <vector>
 
 namespace Script
@@ -32,6 +34,21 @@ namespace Script
  */
 template<typename T> bool FromJSVal(const ScriptRequest& rq, const JS::HandleValue val, T& ret);
 
+template<typename T>
+bool FromJSVal(const ScriptRequest& rq, JS::HandleValue v, std::optional<T>& out)
+{
+	if (v.isNullOrUndefined())
+	{
+		out = std::nullopt;
+		return true;
+	}
+	T value;
+	if (!FromJSVal(rq, v, value))
+		return false;
+	out = std::move(value);
+	return true;
+}
+
 /**
  * Convert a C++ type to a JS::Value. (This might trigger GC. The return
  * value must be rooted if you don't want it to be collected.)

source/simulation2/components/ICmpIdentity.cpp

@@ -1,4 +1,4 @@
-/* Copyright (C) 2022 Wildfire Games.
+/* Copyright (C) 2025 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -21,6 +21,7 @@
 
 #include "simulation2/system/InterfaceScripted.h"
 #include "simulation2/scripting/ScriptComponent.h"
+#include <optional>
 
 
 BEGIN_INTERFACE_WRAPPER(Identity)
@@ -43,7 +44,8 @@ public:
 
 	std::wstring GetCiv() override
 	{
-		return m_Script.Call<std::wstring>("GetCiv");
+		std::optional<std::wstring> civ{m_Script.Call<std::optional<std::wstring>>("GetCiv")};
+		return !civ || !civ.has_value() ? L"" : *civ;
 	}
 };