Check if XRSession exists before adding inputsourceschange listener

[mrxz]

Description:
The tracked-controls system unconditionally adds an inputsourcechange listener when "entering VR". However, the enter-vr event is also used when entering fullscreen mode, in which case an error occurs. This is a regression introduced in fd90434.

This PR simply adds a check to make sure there is an XRSession before adding the event listener.

Changes proposed:

• Check if there's an XRSession before adding inputsourcechange listener in tracked-controls system.

[dmarcos]

Thanks!