BUG: Empty object column writes to parquet as INT32 instead of BINARY L:STRING
- [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

pd.DataFrame({"nullable_col": ["some_val"]}, dtype="object").to_parquet("non-null-obj.parquet")

pd.DataFrame({"nullable_col": []}, dtype="object").to_parquet("null-obj.parquet")
```
Once that runs, `parquet-tools` illustrates the issue. I would expect a datatype such as `OPTIONAL BINARY L:STRING`, as in:
```
$ parquet-tools meta non-null-obj.parquet | grep nullable_col

extra:        pandas = {"index_columns": [{"kind": "range", "name": null, "start": 0, "stop": 1, "step": 1}], "column_indexes": [{"name": null, "field_name": null, "pandas_type": "unicode", "numpy_type": "object", "metadata": {"encoding": "UTF-8"}}], "columns": [{"name": "nullable_col", "field_name": "nullable_col", "pandas_type": "unicode", "numpy_type": "object", "metadata": null}], "creator": {"library": "pyarrow", "version": "1.0.1"}, "pandas_version": "1.1.3"} 
nullable_col: OPTIONAL BINARY L:STRING R:0 D:1
nullable_col:  BINARY SNAPPY DO:4 FPO:32 SZ:80/76/0.95 VC:1 ENC:PLAIN,PLAIN_DICTIONARY,RLE ST:[min: some_val, max: some_val, num_nulls: 0]
```
But instead got a datatype of `OPTIONAL INT32`, as in:

```
$ parquet-tools meta null-obj.parquet | grep nullable_col

extra:        pandas = {"index_columns": [{"kind": "range", "name": null, "start": 0, "stop": 0, "step": 1}], "column_indexes": [{"name": null, "field_name": null, "pandas_type": "unicode", "numpy_type": "object", "metadata": {"encoding": "UTF-8"}}], "columns": [{"name": "nullable_col", "field_name": "nullable_col", "pandas_type": "empty", "numpy_type": "object", "metadata": null}], "creator": {"library": "pyarrow", "version": "1.0.1"}, "pandas_version": "1.1.3"} 
nullable_col: OPTIONAL INT32 R:0 D:1
nullable_col:  INT32 SNAPPY DO:4 FPO:0 SZ:15/14/0.93 VC:0 ENC:PLAIN,RLE,PLAIN_DICTIONARY ST:[no stats for this column]
```

#### Problem description

Writing out a column with pandas type `object` with no values appears to create a parquet type of `INT32` when I would expect it to be `BINARY L:STRING` or similar. I have a daily process that outputs a set of records to parquet and on days where there are no values in an object column the parquet datatype changes to `INT32`, thus breaking my process as the schemas has changed relative to previous days.

#### Expected Output

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

<details>

INSTALLED VERSIONS
------------------
commit           : db08276bc116c438d3fdee492026f8223584c477
python           : 3.7.2.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.6.0
Version          : Darwin Kernel Version 19.6.0: Thu Jun 18 20:49:00 PDT 2020; root:xnu-6153.141.1~1/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : en_US.UTF-8

pandas           : 1.1.3
numpy            : 1.19.2
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.2.3
setuptools       : 50.3.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : 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          : 1.0.1
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
