Fix warning caused by passing null to strpos

[colemanw]

Overview

Fixes a bunch of PHP warnings when you visit the CiviCRM home page (dashboard) without a q= argument in the URL.

Before

Home page looks like this:

After

Home page looks like this:

Technical Details

There was no reason to return NULL in a variable that was supposed to be a string.

[kcristiano]

This looks good and r-run removes the error on php 8.1. Also did an r-run on php 7.4 with no issues.