str accessor functions returns float(NaN) instead of pd.NA
#### Code Sample

```python
import pandas as pd
s = pd.Series(['A', 'B', 'C', 'Aaba', 'Baca', np.nan, 'CABA', 'dog', 'cat'], dtype="string")
type(s.str.lower()[5])  # returns <class 'float'>
```
#### Problem description
the `str` accessor, when working on string-typed series, should return a string-typed series, which should be an array of [`string`, `pd.NA`] only, but it seems that some functions (see list below) can return series that contains `float('nan')`.

#### Affected functions
As of now, I found these str accessor functions to be affected:
`upper` `lower` `replace`
also, `extract(expand=False)` on a string type series returns an object type series, which seems unintended as well.

#### Expected Output
`<class 'pandas._libs.missing.NAType'>`
#### Output of ``pd.show_versions()``

<details>

```
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.0.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.0.0-38-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_IL
LOCALE           : en_IL.UTF-8

pandas           : 1.0.0rc0
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 18.1
setuptools       : 40.8.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None
```
</details>
