BUG: fixed json_normalize for subrecords with NoneTypes (#20030)
TST: additional coverage for the test cases 

DOC: added changes to whatsnew/v0.23.0.txt

Checklist for other PRs (remove this part if you are doing a PR for the pandas documentation sprint):

- [x] closes #20030
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20399?src=pr&el=h1) Report
> Merging [#20399](https://codecov.io/gh/pandas-dev/pandas/pull/20399?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7273ea0709590e6264607f227bb8def0ef656c50?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20399/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/20399?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20399      +/-   ##
==========================================
+ Coverage   91.77%   91.77%   +<.01%     
==========================================
  Files         152      152              
  Lines       49205    49215      +10     
==========================================
+ Hits        45159    45169      +10     
  Misses       4046     4046
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.16% <100%> (ø)` | :arrow_up: |
| #single | `41.84% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20399?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/json/normalize.py](https://codecov.io/gh/pandas-dev/pandas/pull/20399/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vbm9ybWFsaXplLnB5) | `96.93% <100%> (+0.06%)` | :arrow_up: |
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/20399/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvd2luZG93LnB5) | `96.26% <0%> (-0.01%)` | :arrow_down: |
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/20399/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `82.27% <0%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20399/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.11% <0%> (+0.16%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20399?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20399?src=pr&el=footer). Last update [7273ea0...76d2d91](https://codecov.io/gh/pandas-dev/pandas/pull/20399?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback I have updated the PR, let me know if you need me to squash the changes instead. thanks @aerymilts nice patch! keep em coming!