BUG: rolling count on FixedForwardWindowIndexer returns NaN instead of 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.

- [ ] (optional) I have confirmed this bug exists on the master branch 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 pandas.core.window.indexers import FixedForwardWindowIndexer

df = pd.DataFrame([[0, None], [0, None],[0, None],[0, 7]], columns=["a", "b"])
res = df["b"].rolling(window=FixedForwardWindowIndexer(window_size=2),min_periods=0).count()
print(res)
```

#### Problem description

Current output:
0    NaN
1    NaN
2    1.0
3    1.0

#### Expected Output

0    0.0
1    0.0
2    1.0
3    1.0

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

INSTALLED VERSIONS
------------------
commit           : d9fff2792bf16178d4e450fe7384244e50635733
python           : 3.6.9.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.6.13-100.fc30.x86_64
Version          : #1 SMP Fri May 15 00:36:06 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : C.UTF-8
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.0
numpy            : 1.19.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 9.0.3
setuptools       : 45.2.0
Cython           : None
pytest           : 4.6.4
hypothesis       : None
sphinx           : 3.1.2
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.2
html5lib         : None
pymysql          : None
psycopg2         : 2.8.5 (dt dec pq3 ext lo64)
jinja2           : 2.10.1
IPython          : 7.15.0
pandas_datareader: None
bs4              : 4.9.1
bottleneck       : None
fsspec           : 0.7.4
fastparquet      : 0.4.1
gcsfs            : 0.6.2
matplotlib       : 3.3.0
numexpr          : 2.7.1
odfpy            : None
openpyxl         : None
pandas_gbq       : 0.10.0
pyarrow          : 1.0.0
pytables         : None
pyxlsb           : None
s3fs             : 0.2.2
scipy            : 1.3.1
sqlalchemy       : 1.3.18
tables           : 3.6.1
tabulate         : 0.8.7
xarray           : None
xlrd             : None
xlwt             : None
numba            : 0.50.1
</details>
