Passing null to parameter #1 ($string) of type string is deprecated

Problem/Motivation

This module is testing whether a string is empty by running it through `drupal_strlen` and checking if the result is greater than zero. Sending null through this function is deprecated in recent versions of PHP. We can just use `empty` as a replacement.

This error was traced back to this module:

> “Deprecated function: mb_strlen(): Passing null to parameter #1 ($string) of type string is deprecated in drupal_strlen() (line 482 of /code/includes/unicode.inc).”