BUG: ExtensionArray.fillna for scalar values
Closes #20411
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20412?src=pr&el=h1) Report
> Merging [#20412](https://codecov.io/gh/pandas-dev/pandas/pull/20412?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/20412/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20412?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20412      +/-   ##
==========================================
+ 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 | `41.85% <16.66%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20412?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/20412/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2Jhc2UucHk=) | `80.64% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20412/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.11% <100%> (+0.16%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20412?src=pr&el=footer). Last update [7273ea0...6e51710](https://codecov.io/gh/pandas-dev/pandas/pull/20412?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @TomAugspurger 

may want to think about someway of augmenting our is_scalar check that works on EA's. > may want to think about someway of augmenting our is_scalar check that works on EA's

I am not sure that will be possible (or desirable), because in principle such scalars can be anything, including lists and dicts