REGR: Assertion error in 1.2.1 but not 1.0.5 with groupby, resample, min
- [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.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

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

```python
import pandas as pd
from datetime import datetime
from math import nan

def dt(datestr):
    return datetime.strptime(datestr, '%Y-%m-%d')

df = pd.DataFrame(data={'Date_reported': {0: dt('2020-07-23'), 1: dt('2020-07-24'), 2: dt('2020-07-25'), 3: dt('2020-07-26'), 4: dt('2020-07-27'), 5: dt('2020-07-28'), 6: dt('2020-07-29'), 7: dt('2020-07-30'), 8: dt('2020-07-31'), 9: dt('2020-08-01'), 10: dt('2020-08-02'), 11: dt('2020-08-03'), 12: dt('2020-08-04'), 13: dt('2020-08-05'), 14: dt('2020-08-06'), 15: dt('2020-07-23'), 16: dt('2020-07-24'), 17: dt('2020-07-25'), 18: dt('2020-07-26'), 19: dt('2020-07-27'), 20: dt('2020-07-28'), 21: dt('2020-07-29'), 22: dt('2020-07-30'), 23: dt('2020-07-31'), 24: dt('2020-08-01'), 25: dt('2020-08-02'), 26: dt('2020-08-03'), 27: dt('2020-08-04'), 28: dt('2020-08-05'), 29: dt('2020-08-06')}, 'ISO_3_CODE': {0: 'AFG', 1: 'AFG', 2: 'AFG', 3: 'AFG', 4: 'AFG', 5: 'AFG', 6: 'AFG', 7: 'AFG', 8: 'AFG', 9: 'AFG', 10: 'AFG', 11: 'AFG', 12: 'AFG', 13: 'AFG', 14: 'AFG', 15: 'H63', 16: 'H63', 17: 'H63', 18: 'H63', 19: 'H63', 20: 'H63', 21: 'H63', 22: 'H63', 23: 'H63', 24: 'H63', 25: 'H63', 26: 'H63', 27: 'H63', 28: 'H63', 29: 'H63'}, 'Cumulative_cases': {0: 35915, 1: 35981, 2: 36036, 3: 36157, 4: 36263, 5: 36368, 6: 36471, 7: 36542, 8: 36542, 9: 36710, 10: 36710, 11: 36710, 12: 36747, 13: 36829, 14: 36896, 15: 47194, 16: 47856, 17: 48448, 18: 48952, 19: 49388, 20: 49825, 21: 50409, 22: 51000, 23: 51380, 24: 51942, 25: 52265, 26: 52490, 27: 52771, 28: 53457, 29: 53877}, 'Regional_office': {0: 'ROAP', 1: 'ROAP', 2: 'ROAP', 3: 'ROAP', 4: 'ROAP', 5: 'ROAP', 6: 'ROAP', 7: 'ROAP', 8: 'ROAP', 9: 'ROAP', 10: 'ROAP', 11: 'ROAP', 12: 'ROAP', 13: 'ROAP', 14: 'ROAP', 15: nan, 16: nan, 17: nan, 18: nan, 19: nan, 20: nan, 21: nan, 22: nan, 23: nan, 24: nan, 25: nan, 26: nan, 27: nan, 28: nan, 29: nan}})
result = df.groupby(['ISO_3_CODE']).resample('W', on='Date_reported').min()
```

#### Problem description

When running it in 1.21 it fails with:

```
    assert len(result._mgr.blocks) == 1
AssertionError
```

That exception is raised from another:

```
NotImplementedError: function is not implemented for this dtype: [how->min,dtype->object]
```

#### Expected Output
When running the above code in 1.05 it works without error. 

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

<details>

INSTALLED VERSIONS
------------------
commit           : 9d598a5e1eee26df95b3910e3f2934890d062caa
python           : 3.8.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.8.0-40-generic
Version          : #45~20.04.1-Ubuntu SMP Fri Jan 15 11:35:04 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_NZ.UTF-8
LOCALE           : en_NZ.UTF-8
pandas           : 1.2.1
numpy            : 1.19.4
pytz             : 2020.5
dateutil         : 2.8.1
pip              : 20.3.3
setuptools       : 51.1.1
Cython           : None
pytest           : 6.2.1
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : 1.1
pymysql          : None
psycopg2         : 2.8.6 (dt dec pq3 ext lo64)
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : 3.0.5
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : 1.3.22
tables           : None
tabulate         : None
xarray           : None
xlrd             : 2.0.1
xlwt             : None
numba            : None

</details>
