Bug 6291 - Fix bugs of new Restconf 11 implementation

Bug 6291 - Fix bugs of new Restconf 11 implementation

- first identifier does not start with slash
when deserialization is invoked
- adapted unit tests
- return error 404 when data missing with GET
operation
- return config data when state data does
not exists with GET operation
- return state data when config data does
not exists with GET operation
- identifier can be null and it is deserialized to
YangInstanceIdentifier.EMPTY

Change-Id: I426a2bc1cb0e908dc189553e464f405ef4ce1457
Signed-off-by: Ivan Hrasko <ihrasko@cisco.com>