Fixed null reference exception in Token operator overload

[vlada-shubina]

Problem:

//given  Token? token = null; if (token == default || token == null) { //leads to null ref exception }

Added check for null in Token operator overload.
Workaround: use token is null instead.

[KalleOlaviNiemitalo]

Fixes #1797, thus competes with #1802

[jonsequitur]

Thanks, @vlada-shubina!