Allow JSON serialization to work with json.dumps

[hoxbro]

Fixes #578

The changes in #582 made it possible to round-trip serialize <-> deserialize within param.

A completely valid way to deserialize is to have the serialized values created outside param like this:

import json
import param

class User(param.Parameterized):
    A = param.Date(default=None)

data = json.dumps({"A": None})    # gives: '{"A": null}' <- note that null is not a string
User.param.deserialize_parameters(data)

Which raised this error before these changes:

Traceback (most recent call last):
  File "/home/shh/miniconda3/envs/holoviz/lib/python3.9/site-packages/IPython/core/interactiveshell.py", line 3378, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "/tmp/ipykernel_70470/342607310.py", line 8, in <module>
    User.param.deserialize_parameters(data)
  File "/home/shh/Development/holoviz/repos/param/param/parameterized.py", line 2121, in deserialize_parameters
    return serializer.deserialize_parameters(self_or_cls, serialization, subset=subset)
  File "/home/shh/Development/holoviz/repos/param/param/serializer.py", line 113, in deserialize_parameters
    deserialized = pobj.param[name].deserialize(value)
  File "/home/shh/Development/holoviz/repos/param/param/__init__.py", line 1954, in deserialize
    return dt.datetime.strptime(value, "%Y-%m-%dT%H:%M:%S.%f")
TypeError: strptime() argument 1 must be str, not None

[philippjfr]

@hoxbro Your fixes here look good, the issue I noted above precede you. So to finish this I'd like to ask you to change all serialize implementations to return None if the value is None and for all deserialize implementations to handle None as input.

[philippjfr]

Okay, sorry again. I think you already did that. Sorry for the noise.