Nano: don't generate accessor methods for nested methods

For nested message objects, don't generate accessor methods because they have
a default value that is not a valid value (null), so there is no reason to have
get/set/has/clear methods for them. Clients and protos (while serializing) can
check against the invalid value to see if it's been set.

Change-Id: Ic63400889581271b8cbcd9c45c84519d4921fd4b