BUG: Grouper should have a dropna attribute
- [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.

---

xref #35612

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

```python
In [1]: import pandas as pd 
   ...: df = pd.DataFrame({"A": [0, 0, 1, None], "B": [10, 2, 10, None]}) 
   ...: gb = df.groupby('A', dropna=False)                                     

In [2]: gb.dropna                                                              
Out[2]: False

In [3]: gb.grouper.dropna                                                      
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-3-4d8081f9c78e> in <module>
----> 1 gb.grouper.dropna

AttributeError: 'BaseGrouper' object has no attribute 'dropna'
```

#### Problem description

Now that we support a `dropna` argument to `DataFrameGroupBy` and `SeriesGroupBy` we need to be able to use it in `Grouper` (#35751)

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

<details>


INSTALLED VERSIONS
------------------
commit           : f34a56b4ddcf51a8a03a9d20f07ab039f64ff2e6
python           : 3.7.7.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.4.0-48-generic
Version          : #52-Ubuntu SMP Thu Sep 10 10:58:49 UTC 2020
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : C.UTF-8
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.0.dev0+484.gf34a56b4d
numpy            : 1.18.5
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 45.2.0.post20200210
Cython           : 0.29.21
pytest           : 5.4.3
hypothesis       : 5.19.0
sphinx           : 3.1.1
blosc            : None
feather          : None
xlsxwriter       : 1.2.9
lxml.etree       : 4.4.1
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.16.1
pandas_datareader: None
bs4              : 4.9.1
bottleneck       : 1.3.2
fsspec           : 0.7.4
fastparquet      : 0.4.0
gcsfs            : 0.6.2
matplotlib       : 3.2.1
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.4
pandas_gbq       : None
pyarrow          : 0.16.0
pytables         : None
pyxlsb           : None
s3fs             : 0.4.2
scipy            : 1.5.0
sqlalchemy       : 1.3.18
tables           : 3.6.1
tabulate         : 0.8.7
xarray           : 0.15.1
xlrd             : 1.2.0
xlwt             : 1.3.0
numba            : 0.50.1

</details>
