Fix JSONDecodeError for intfstat

[ZhaohuiS]

What I did

Fix #14089.

PR #2636 introduced the following error if I run intfstat -c twice.
The first run is fine, because there is no intfstat tmp file, but the twice run will save new data into same tmp file again, this time, it calls json.dump(data, open(cnstat_fqn_file, 'w')) and throws traceback.

admin@str2-7050cx3-acs-01:~$ intfstat -c                    
Cleared counters
admin@str2-7050cx3-acs-01:~$ intfstat -c
Traceback (most recent call last):
  File "/usr/local/bin/intfstat", line 376, in <module>
    main()
  File "/usr/local/bin/intfstat", line 324, in main
    json.dump(data, open(cnstat_fqn_file, 'w'))
  File "/usr/lib/python3.9/json/__init__.py", line 179, in dump
    for chunk in iterable:
  File "/usr/lib/python3.9/json/encoder.py", line 431, in _iterencode
    yield from _iterencode_dict(o, _current_indent_level)
  File "/usr/lib/python3.9/json/encoder.py", line 405, in _iterencode_dict
    yield from chunks
  File "/usr/lib/python3.9/json/encoder.py", line 438, in _iterencode
    o = _default(o)
  File "/usr/lib/python3.9/json/encoder.py", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type datetime is not JSON serializable

How I did it

The default value for the default parameter is None.
Should use json_serial as default format.

How to verify it

run intfstat -c twice

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

[StormLiangMS]

@qiluo-msft could you help to review?

[StormLiangMS]

LGTM

[ZhaohuiS]

#2746 will revert #2636.

[ZhaohuiS]

Close it, since #2746 fixed #2739.