Return nil when there are no usable keys

The old behavior resulted in a call to `empty?` on a `Puppet::Util::Log`
instance on line 38 when queried from puppet. This would throw an
exception, causing catalog compilation to fail completely rather than
the expected result of a fallback to other hiera backends.
Merge pull request #16 from envato/allow-fallback-after-decrypt-failures

Fix Puppet queries when no usable keys are available