Added Nonnull annotation for get operation

Added Nonnull annotation for get operation in NodeContextSupplier to harden contract and
preventing of retrieving null reference from get method.

Change-Id: Ibcd8e601d5eec2bf192f8d3c779c7bab668d3a26
Signed-off-by: Lukas Sedlak <lsedlak@cisco.com>
Merge "Added Nonnull annotation for get operation"