Revert "Never return a null metadata object. In the case that we try to rad the metadata on anything else than a container or a data object, return an UnsupportedOperationException."

This reverts commit 92fea8feecc74a93664031513c79aafec5c3fecb.
Throw an UnsupportedOperationException when trying to read the metadata
of anything else than a container or a data object, rather than
returning null.

Closes #3.