Description

In jQuery 3.6.0, $element.css("--custom") correctly returned undefined for a custom property that isn’t set. But in jQuery 3.6.1, it crashes with TypeError: ret is undefined.

This breaks the Zulip settings dialog.

This was presumably introduced by #4930 (cc @fecore1 @mgol).

Link to test case

https://codepen.io/anderskaseorg/pen/rNvNZWW