Avoid `Type error: 'searchParams' is possibly 'null'.` in example code

[unitof]

Out of the box (on next@14), the app router example code fails to build with a Type error, since searchParams can be null:

./app/Fathom.tsx:25:23
Type error: 'searchParams' is possibly 'null'.
  23 |
  24 |     trackPageview({
> 25 |       url: pathname + searchParams.toString(),
     |                       ^
  26 |       referrer: document.referrer
  27 |     });
  28 |   }, [pathname, searchParams]);
Error: Command "npm run vercel-build" exited with 1

This allows for that with the optional chaining ?. operator.

[derrickreimer]

Thanks @unitof!