BUG: Fix isna cannot handle ambiguous typed list
- [x] closes #20675
- [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/20971?src=pr&el=h1) Report
> Merging [#20971](https://codecov.io/gh/pandas-dev/pandas/pull/20971?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/587a0ddf99d71d3fa3643ab3c53249f2998a798b?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `50%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20971/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/20971?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20971      +/-   ##
==========================================
- Coverage   91.82%   91.81%   -0.01%     
==========================================
  Files         153      153              
  Lines       49483    49487       +4     
==========================================
  Hits        45437    45437              
- Misses       4046     4050       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.21% <50%> (-0.01%)` | :arrow_down: |
| #single | `41.85% <33.33%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20971?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/missing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20971/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL21pc3NpbmcucHk=) | `91.95% <50%> (-0.99%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20971/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.38% <0%> (-0.21%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20971?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/20971?src=pr&el=footer). Last update [587a0dd...c2c45b1](https://codecov.io/gh/pandas-dev/pandas/pull/20971?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks @jreback, fixed! thanks @Licht-T 