GameSettings: ensure savegameID is either a file name or undefined.

The current code could end up with savegameID being undefined, false, a filename string, or null depending on the code path taken.
Adapt C++ in consequence.

binaries/data/mods/public/gamesettings/GameSettings.js

@@ -20,7 +20,7 @@ class GameSettings
 		});
 
 		Object.defineProperty(this, "savegameID", {
-			"value": !!savegameID,
+			"value": savegameID,
 		});
 
 		// Load all possible civ data - don't presume that some will be available.

binaries/data/mods/public/gui/gamesetup/SetupWindow.js

@@ -30,7 +30,7 @@ class SetupWindow
 
 		const mapCache = new MapCache();
 		g_GameSettings = new GameSettings();
-		g_GameSettings.init(mapCache, g_IsController ? savedGame : null);
+		g_GameSettings.init(mapCache, g_IsController ? savedGame : undefined);
 
 
 		let netMessages = new NetMessages();

binaries/data/mods/public/gui/gamesetup_mp/gamesetup_mp.js

@@ -319,7 +319,7 @@ async function handleAuthenticated(message, loadSavedGame)
 	}
 	g_IsConnecting = false;
 
-	const savegameID = loadSavedGame ? await Engine.PushGuiPage("page_loadgame.xml") : null;
+	const savegameID = loadSavedGame ? await Engine.PushGuiPage("page_loadgame.xml") : undefined;
 
 	if (loadSavedGame && !savegameID)
 	{
@@ -329,7 +329,7 @@ async function handleAuthenticated(message, loadSavedGame)
 	}
 
 	Engine.SwitchGuiPage("page_gamesetup.xml", {
-		"savedGame": savegameID ?? message.savedGame,
+		"savedGame": savegameID, // Undefined or the savegame ID
 		"serverName": g_ServerName,
 		"hasPassword": g_ServerHasPassword
 	});

source/network/scripting/JSInterface_Network.cpp

@@ -283,7 +283,7 @@ void StartNetworkGame(const ScriptInterface& scriptInterface, JS::HandleValue sa
 	JS::RootedValue attribs(rq.cx, attribs1);
 	std::string attributesAsString{Script::StringifyJSON(rq, &attribs)};
 
-	if (savegame.isFalse())
+	if (savegame.isUndefined())
 	{
 		g_NetClient->SendStartGameMessage(attributesAsString);
 		return;