BUG: Unable to set StringArray for DataFrame with single column and dtype string
### 

- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the [latest version](https://pandas.pydata.org/docs/whatsnew/index.html) of pandas.

- [ ] I have confirmed this bug exists on the master branch of pandas.


### Reproducible Example

```python
import pandas as pd


df_single = pd.DataFrame(data={"a": [None, "two"]}, index=[0, 1], dtype="string")
df_multi = pd.DataFrame(data={"a": [None, "two"], "b":["1", "2"]}, index=[0, 1], dtype="string")


# ok
df_single.loc[0, ["a"]] = "one"

# ok
df_multi.loc[0, ["a"]] = "one"

# not ok
df_single.loc[0, ["a"]] = ["one"]

# ok
df_multi.loc[0, ["a"]] = ["one"]

# not ok
df_single.loc[0, ["a"]] = pd.array(["one"], dtype="string")

# ok
df_multi.loc[0, ["a"]] = pd.array(["one"], dtype="string")
```


### Issue Description

When attempting to set a single value (string array) into an DataFrame with single column and dtype string, exception is raised: `ValueError: setting an array element with a sequence.`

The code example shows same operation performed different ways with a DataFrame with single and multiple columns. No issue when DataFrame has more than one column.

### Expected Behavior

Set the value as in DataFrame with multiple columns.

### Installed Versions

<details>

INSTALLED VERSIONS
------------------
commit           : 945c9ed766a61c7d2c0a7cbb251b6edebf9cb7d5
python           : 3.8.10.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.19043
machine          : AMD64
processor        : Intel64 Family 6 Model 142 Stepping 12, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : Norwegian Bokmål_Norway.1252

pandas           : 1.3.4
numpy            : 1.20.3
pytz             : 2021.3
dateutil         : 2.8.2
pip              : 21.1.3
setuptools       : 58.2.0
Cython           : None
pytest           : 6.2.5
hypothesis       : None
sphinx           : 4.0.2
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 3.0.2
IPython          : 7.28.0
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 5.0.0
pyxlsb           : None
s3fs             : None
scipy            : 1.7.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : 0.54.0

</details>
