ENH: Serialize null interface as literal

[fumitoh]

When Cells and Spaces are deleted, Their interfaces _impl point to the NullImpl object.
When a reference refers to deleted Cells and Spaces, These objects are pickled when the model is serialized.

Here is a sample script to reproduce the phenomenon.

import modelx as mx

@mx.defcells
def foo():
    return 1

mx.Model1.Space1.bar = foo

del mx.Model1.Space1.foo

mx.Model1.write('Ref2Deleted') 

The resulting file Ref2Deleted/Space1/.__init__.py looks like below

# modelx: pseudo-python
# This file is part of a modelx model.
# It can be imported as a Python module, but functions defined herein
# are model formulas and may not be executable as standard Python.

from modelx.serialize.jsonvalues import *

_formula = None

_bases = []

_allow_none = None

_spaces = []

# ---------------------------------------------------------------------------
# References

bar = ("Pickle", 2229981096848)

Saving the null object as binary is not transparent, so I want to have it serialized literally in the touple representing the ref's value.