Fix error when windowId is undefined in Wayland

This is a reimplementation of Bismuth-Forge#490
My esbuild and tsc on Debian Testing right now aren't smart enough to
convert the ?. optional chaining into something that Qt 5.15 can
understand.

src/kwinscript/driver/window.ts

@@ -292,9 +292,11 @@ export class DriverWindowImpl implements DriverWindow {
 
   public toString(): string {
     // Using a shorthand name to keep debug message tidy
-    return `KWin(${this.client.windowId.toString(16)}.${
-      this.client.resourceClass
-    })`;
+    var windowIdString = 'undefined'
+    if (this.client.windowId) {
+      windowIdStirng = this.client.windowId.toString(16)
+    }
+    return `KWin(${windowIdString}.${this.client.resourceClass})`;
   }
 
   public visibleOn(surf: DriverSurface): boolean {