Combining two Series of dtype `Int8` converts them to `Int64`
#### Code Sample, a copy-pastable example if possible

```python
>>> s1 = pd.Series([91, 92, pd.NA, 94], dtype='Int8')                                                       
>>> s2 = s1.copy()                                                                                          

>>> s1 + s2  # result looks correct!                                                                                                  
0     -74
1     -72
2    <NA>
3     -68
dtype: Int8

>>> s1.combine(s2, lambda x, y: x+y)  # dtype has changed!                                                       
0     -74
1     -72
2    <NA>
3     -68
dtype: Int64
```
#### Problem description

The dtype has changed.

#### Expected Output
```
0     -74
1     -72
2    <NA>
3     -68
dtype: Int8
```

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : 87b149f41d305fefa2a22b379b915c33b24a6d60
python           : 3.7.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-76-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 0.26.0.dev0+2024.g87b149f41
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 45.1.0.post20200119
Cython           : 0.29.14
pytest           : 5.3.5
hypothesis       : 5.5.1
sphinx           : 2.3.1
blosc            : None
feather          : None
xlsxwriter       : 1.2.7
lxml.etree       : 4.5.0
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.12.0
pandas_datareader: None
bs4              : 4.8.2
bottleneck       : 1.3.1
fastparquet      : 0.3.3
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : 3.1.3
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.1
pandas_gbq       : None
pyarrow          : 0.16.0
pytables         : None
pytest           : 5.3.5
pyxlsb           : None
s3fs             : 0.4.0
scipy            : 1.4.1
sqlalchemy       : 1.3.13
tables           : 3.6.1
tabulate         : 0.8.6
xarray           : 0.15.0
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.7
numba            : 0.48.0

</details>
