Return None if no active window

Hi, this is really cool!
I'm using it for one of my open-source projects called Textylic: https://akhilesh-balaji.github.io/Textylic/

However, if there is no active window detected, it returns an empty string (`pygetwindow.getActiveWindow()` method). This is really annoying, because sometimes, there really are windows without no title, which get bundled in with that. So, when there really is an active window, my program thinks there is non, because it returns an empty string.

If there was no active window, it would be nice if the method could return `None`, or if there was a parameter where I could specify what I wanted returned.