Fixes NPE during version range resolution

The NexusWorkspaceReader returned null on findVersions() instead of
an empty collection. This is in violation with the javadoc and
resulted in NPE's during version range resolutions.
Merge pull request #751 from thallgren/master

Fix for NPE during version range resolution.
Merge remote-tracking branch 'origin/master' into nexus-5472-proxy404