maint: SSL::Inventory.serial should report missing names.

Our SSL inventory was able to find the serial number of a certificate by name,
but was incapable of living up to the contract it offered, that it would
actually report when a certificate was missing.

Now it returns nil, which is the same case as "no inventory", if the
certificate was not found, rather than accidentally returning the entire
inventory data as raw strings.

Reviewed-By: Pieter van de Bruggen <pieter@puppetlabs.com>