VerifyExpiresAt returns false when not required and exp is empty

[nrgraham23]

The VerifyExpiresAt function returns false when it is not required and the exp token is an empty string because it returns false if there is not a match in the switch statement: https://github.com/golang-jwt/jwt/blob/main/map_claims.go#L58

Based on the comment on the method and the old version of this method, it seems like this is a bug.