Add nonnullFoo() getters for non-semantic objects

Add nonnullFoo() getters for non-semantic objects

Add non-null getters for containers without presence statement. They
return a lazy-initialized containers' empty instances in case the
container is null.

Non-null getters are implemented in Builder's inner implementation
of the container's parent.

Container's empty instance is located in container's builder class
and is hidden behind a static method.

In the binding codec part a nonnull method is generated using usual
lazy proxy backed by an empty ContainerNode.

JIRA: MDSAL-673
Change-Id: I3a5a346b32b89e7c28cad31536593824dee3c232
Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>
Signed-off-by: Samuel Schneider <samuel.schneider@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>