allow load plugin result with empty loader to have null contents

[clydin]

Currently, a plugin that returns a loader value of empty also requires the contents field to be set to a non-null value for the loader to be used. Since the provided contents will never be used in the empty loader case, the contents field is now no longer required in the case when an empty loader is returned. This removes the chance of a plugin unintentionally not using the returned empty loader.