869378 - API does not list providedProducts for custom content

Bug https://bugzilla.redhat.com/show_bug.cgi?id=869378 shows that Provided products are not set for custom products.

Provided products are just marketing products which are not used for custom subscriptions. We could either hide this field for this case, or put N/A there. This patch does the latter, because the way we structure our python code does not allow hiding individual fields.