Sensible defaults for Servlet & Filter registrations in mock

Prior to this commit, the getter methods in MockServletContext threw an
UnsupportedOperationException when trying to retrieve Servlet and
Filter registrations.

This commit improves the behavior of these methods by returning null
when a single registration is requested and an empty map when all
registrations are requested. This is now in line with the Javadoc for
ServletContext. Note, however, that the corresponding setter methods
still throw UnsupportedOperationExceptions which is suitable behavior
for a mock.

Issue: SPR-12290