ManagementView.getComponentsForType(type) returns null, rather than an empty Set, if no Components of the specified type are deployed

This is a public-facing API so should return an empty Set rather than null to make it less error-prone and easier to use. There are a bunch of other methods that return Collections in the ManagementView interface - these should also be reworked to never return null. The Javadoc should also indicate the methods will never return null.