Subscriptions: Don't display Subscription Management link in Account when no subscription products are available

[fjbender]

Currently the link to the subscription management from the customer account is always displayed for logged-in customers, see https://github.com/mollie/Shopware6/blob/0c6936170b5ebc041b7dcfd791f528a0dce3a962/src/Resources/views/storefront/page/account/sidebar.html.twig#L3-L14

For consumers of merchants not offering subscription products this might lead to confusion. The link should not be visible either when no subscription products are available in the shop (automatically), or a respective toggle has not been set in Admin (manually).