Fix deprecation notice for passing null to preg_replace

[kissit]

We've been seeing the following warning in our logs:

Severity: 8192 --> preg_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated /var/www/somoplatform/current/system/core/Common.php 730

This PR fixes the problem. For reference, the root cause is calling $this->security->xss_clean() passing either a null or an array that has one or more null values.