BUG: Regression on SeriesGrouper using Timestamp index with pandas 1.3.0
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

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

---


#### Code Sample, a copy-pastable example

```python
import pandas as pd
import numpy as np


def agg(series):
    if series.isna().values.all():
        return None
    return np.sum(series)


df = pd.DataFrame([1.0], index=[pd.Timestamp("2018-01-16 00:00:00+00:00")])
df.groupby(lambda x: 1).agg(agg)
```

#### Problem description

This raise with 1.3.0 and succeeded with 1.2.5

```
Traceback (most recent call last):
  File "/tmp/t.py", line 12, in <module>
    df.groupby(lambda x: 1).agg(agg)
  File "/home/phil/src/pandas/pandas/core/groupby/generic.py", line 1010, in aggregate
    result = gba.agg()
  File "/home/phil/src/pandas/pandas/core/apply.py", line 164, in agg
    return self.agg_list_like()
  File "/home/phil/src/pandas/pandas/core/apply.py", line 355, in agg_list_like
    new_res = colg.aggregate(arg)
  File "/home/phil/src/pandas/pandas/core/groupby/generic.py", line 249, in aggregate
    ret = self._aggregate_multiple_funcs(func)
  File "/home/phil/src/pandas/pandas/core/groupby/generic.py", line 303, in _aggregate_multiple_funcs                                                                                        
    results[key] = self.aggregate(func)
  File "/home/phil/src/pandas/pandas/core/groupby/generic.py", line 265, in aggregate
    return self._python_agg_general(func, *args, **kwargs)
  File "/home/phil/src/pandas/pandas/core/groupby/groupby.py", line 1310, in _python_agg_general                                                                                             
    result = self.grouper.agg_series(obj, f)
  File "/home/phil/src/pandas/pandas/core/groupby/ops.py", line 1028, in agg_series
    result = self._aggregate_series_fast(obj, func)
  File "/home/phil/src/pandas/pandas/core/groupby/ops.py", line 1053, in _aggregate_series_fast                                                                                              
    result, _ = sgrouper.get_result()
  File "pandas/_libs/reduction.pyx", line 281, in pandas._libs.reduction.SeriesGrouper.get_result                                                                                            
    cached_series, cached_index, islider, vslider)
  File "pandas/_libs/reduction.pyx", line 88, in pandas._libs.reduction._BaseGrouper._apply_to_group                                                                                         
    res = self.f(cached_series)
  File "/home/phil/src/pandas/pandas/core/groupby/groupby.py", line 1296, in <lambda>
    f = lambda x: func(x, *args, **kwargs)
  File "/tmp/t.py", line 6, in agg
    if series.isna().values.all():
  File "/home/phil/src/pandas/pandas/core/series.py", line 5161, in isna
    return generic.NDFrame.isna(self)
  File "/home/phil/src/pandas/pandas/core/generic.py", line 7145, in isna
    return isna(self).__finalize__(self, method="isna")
  File "/home/phil/src/pandas/pandas/core/dtypes/missing.py", line 138, in isna
    return _isna(obj)
  File "/home/phil/src/pandas/pandas/core/dtypes/missing.py", line 177, in _isna
    result, index=obj.index, name=obj.name, copy=False
  File "/home/phil/src/pandas/pandas/core/series.py", line 430, in __init__
    com.require_length_match(data, index)
  File "/home/phil/src/pandas/pandas/core/common.py", line 528, in require_length_match
    "Length of values "
ValueError: Length of values (1) does not match length of index (0)
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : c8b79e1bd30fb4d36e5c1f894757d59c57837528
python           : 3.7.3.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.19.0-17-amd64
Version          : #1 SMP Debian 4.19.194-2 (2021-06-21)
machine          : x86_64
processor        :
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8
pandas           : 1.3.0+4.gc8b79e1bd3
numpy            : 1.21.0
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.1.3
setuptools       : 57.1.0
Cython           : 0.29.23
pytest           : 6.2.4
hypothesis       : 6.14.1
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None


</details>
