Should I use null to see if a variable is empty in PHP?

[Kwah]

Which is better to use when I use _GET['something here'] for a variable to check if it is empty or not\[code\]if (isset($_GET['url']) != '') { //do stuff with it}\[/code\]OR\[code\]if (isset($_GET['url']) != NULL) { //do stuff with it}\[/code\]'' or null or something else?Please don't call this over optimizing or micro optimizing, I am simply looking for best practices, thank you