`JwtAuthenticationProvider` `authenticate` nullability fix

[alfonsoristorato]

Addressing issue I created https://github.com/spring-projects/spring-security/issues/18229. Not sure if this was ever intended to be nullable, but as we throw when token is null it makes sense to me to add @NotNull

No tests added as this is already covered in https://github.com/spring-projects/spring-security/blob/f6cb0bd610ec5cc1739f26f438505ffbf2da5c12/oauth2/oauth2-resource-server/src/test/java/org/springframework/security/oauth2/server/resource/authentication/JwtAuthenticationProviderTests.java#L157