nextjs: simplify check in useEffect (#3)

- Remove check for undefined
- Change the cleanup function to not return anything (in-line with useEffect typing)

Author: divyanshu013

examples/nextjs/app/page.js

@@ -12,17 +12,18 @@ export default function App() {
   useEffect(() => {
     const container = containerRef.current;
 
-    if (container && typeof PSPDFKit !== "undefined" && window.PSPDFKit) {
-      const { PSPDFKit } = window;
-
+    const { PSPDFKit } = window;
+    if (container && PSPDFKit) {
       PSPDFKit.load({
         container,
         document: "/example.pdf",
         baseUrl: `${window.location.protocol}//${window.location.host}/`,
       });
     }
 
-    return () => PSPDFKit?.unload(container);
+    return () => {
+      PSPDFKit?.unload(container);
+    };
   }, []);
 
   return (