Fix: Missing events claim check on backchannel logout

[SamuelWei]

https://openid.net/specs/openid-connect-backchannel-1_0.html#LogoutToken

2.4. Logout Token
The following Claims are used within the Logout Token:

[...]
events
REQUIRED. Claim whose value is a JSON object containing the member name http://schemas.openid.net/event/backchannel-logout. This declares that the JWT is a Logout Token. The corresponding member value MUST be a JSON object and SHOULD be the empty JSON object {}.
[...]

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