Empty lists get stripped from package dicst in package_show()

[seanh]

If there's an IDatasetForm plugin active with a form_to_db_schema(), then packages with e.g. no extras get the 'extras' key entirely stripped by the validation in package_show, which causes some tests to crash, e.g. in ckan/tests/functional/api/test_activity.py (if the tests are run with a suitable IDatasetForm plugin active).

If a package has no extras the dict returned by package_show should still have an 'extras' key with an empty list as the value.

Suspect this also effects packages with no tags, no groups, etc.