[mediacapture-main] getCapabilities() cannot return null

jan-ivar has just created a new issue for https://github.com/w3c/mediacapture-main:

== getCapabilities() cannot return null ==
[getCapabilities](https://w3c.github.io/mediacapture-main/getusermedia.html#dom-inputdeviceinfo-getcapabilities) says at the end: *"then this method returns null."*, but the WebIDL return value isn't nullable.

Chrome returns an empty dictionary, which I think makes sense.

Please view or discuss this issue at https://github.com/w3c/mediacapture-main/issues/538 using your GitHub account