BUG: read_csv converts NaN to 1.0 in certain circumstances
- [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.

---

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

```python
import pandas as pd
import numpy as np
a = pd.DataFrame([np.nan, True, False])
a.to_csv('test.csv',index=False)
b = pd.read_csv('test.csv',dtype='float')
print(b)

```
Output:
```
     0
0  1.0
1  1.0
2  0.0
```

#### Problem description

When reading a csv file containing NaNs mixed with booleans, and when told to convert to a float, pandas converts NaN to 1.0.  

#### Expected Output

I'd expect NaN to be read as NaN.  It should behave the same as when I do this:
```python
print(a.astype('float'))
```
Output:
```
     0
0  NaN
1  1.0
2  0.0
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : 2cb96529396d93b46abab7bbc73a208e708c642e
python           : 3.8.8.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.14.232-123.381.amzn1.x86_64
Version          : #1 SMP Wed May 19 18:00:09 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.4
numpy            : 1.20.2
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.1
setuptools       : 49.6.0.post20210108
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.3
IPython          : 7.22.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : 2021.04.0
fastparquet      : None
gcsfs            : None
matplotlib       : 3.4.1
numexpr          : 2.7.3
odfpy            : None
openpyxl         : 3.0.7
pandas_gbq       : None
pyarrow          : 4.0.0
pyxlsb           : None
s3fs             : 2021.04.0
scipy            : 1.6.3
sqlalchemy       : 1.4.11
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : 0.53.1

</details>
