Fix and test scalar extension dtype op corner case
Fixes the following behavior in master:

```
ser = pd.Series(['a', 'b', 'c'])

>>> ser + "category"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pandas/core/ops.py", line 1233, in wrapper
    return dispatch_to_extension_op(op, left, right)
  File "pandas/core/ops.py", line 1163, in dispatch_to_extension_op
    res_values = op(new_left, new_right)
TypeError: can only concatenate list (not "str") to list


>>> ser + "Int64"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pandas/core/ops.py", line 1233, in wrapper
    return dispatch_to_extension_op(op, left, right)
  File "pandas/core/ops.py", line 1163, in dispatch_to_extension_op
    res_values = op(new_left, new_right)
TypeError: can only concatenate list (not "str") to list
```

and the same for the reversed ops.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22378?src=pr&el=h1) Report
> Merging [#22378](https://codecov.io/gh/pandas-dev/pandas/pull/22378?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/70e6f7c3ce7aca9a0ee08bacb2fe0ad85db02d88?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22378/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/22378?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22378   +/-   ##
=======================================
  Coverage   92.05%   92.05%           
=======================================
  Files         169      169           
  Lines       50709    50709           
=======================================
  Hits        46679    46679           
  Misses       4030     4030
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.46% <ø> (ø)` | :arrow_up: |
| #single | `42.25% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22378?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/22378/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `96.71% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22378?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/22378?src=pr&el=footer). Last update [70e6f7c...a1e2042](https://codecov.io/gh/pandas-dev/pandas/pull/22378?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jorisvandenbossche think this is mergeable?  I'm looking through the queue for stuff sufficiently small/easy to take off @jreback's plate. Yes, merged, thanks!