FwdRef and FwdPtr: Nullness and Availability are two different things

@gpetruc @rappoccio
Fix FwdRef so that at get() time it checks Availability rather than Nullness.
Should fix the access when the original collection is kept and the forward pointed is dropped.
This way should also be complaint with the implementation of edm::FwdRef::isAvailable that return true if any of the two collections is available. With old implementation despite isAvailable being true, dereferencing would throw if the fwd pointed was dropped.

See:
https://hypernews.cern.ch/HyperNews/CMS/get/edmFramework/3282.html

Similar to PR #3850