test: adds a test for missing blocks finalize

This patch adds a test for finalizing a file when blocks have been
assigned to a file but not registered.
fix: finalize with unregistered blocks (cass)

This patch fixes the scenario when blocks have been assigned to a file
but not registered. The driver should treat the blocks as though they do
not exist at all.
fix: finalize w/ unregistered blocks (mongo)

This patch fixes an issue with the mongo driver when trying to finalize
a file with blocks that have not been registered.