* [ONNX] Add null node.

Optional inputs in ONNX standard are represented by empty string.
We need a placeholder to keep information which inputs are not provided.

* Rename class null_node -> NullNode

* Remove unnecesary validate_and_infer_types method

* Add <memory> header

* Change name != "" -> !name.empty()

* Change constructor

* Little description

* Change node type in NullNode

* Add is_null() method

* Docstring

* Add UT

* Use override

* Style check

* Update null_node.cpp