OIDC Authenticator will try to synchronize groups even if no mapping is defined

This issue comes from the fact that #isGroupSync() will return true as soon as a group claim exists (https://github.com/xwiki-contrib/oidc/blob/8e624ade09a6a10b9e695d16d90762c311504b2a/oidc-authenticator/src/main/java/org/xwiki/contrib/oidc/auth/internal/OIDCClientConfiguration.java#L1261) without checking if the mapping actually exists (https://github.com/xwiki-contrib/oidc/blob/8e624ade09a6a10b9e695d16d90762c311504b2a/oidc-authenticator/src/main/java/org/xwiki/contrib/oidc/auth/internal/OIDCClientConfiguration.java#L1276)