set_type_codec encoder to handle numpy/pandas null types not working with copy_records_to_table

Chưa có ai nhận issue này.

* **asyncpg version**:0.21.0
* **PostgreSQL version**: PostgreSQL 13.1 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-44), 64-bit
* **Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?**:
* **Python version**: 3.7
* **Platform**: Ubuntu 18.04
* **Do you use pgbouncer?**: no
* **Did you install asyncpg with pip?**: yes
* **If you built asyncpg locally, which version of Cython did you use?**: -
* **Can the issue be reproduced under both asyncio and
[uvloop](https://github.com/magicstack/uvloop)?**: -

I am trying to set up an asyncpg in order to use ```copy_records_to_table``` method passing pandas data frame, converted to list of lists.

My table is simple:
```sql
create table sg.test("no" int4, dt timestmap)
```

As pandas/numpy have their own Null types for different types (pd.NaT for dates and np.nan for numerics) I tried to set up an appropriate encoder:

```python
import asyncio
import asyncpg
import pandas as pd
import numpy as np

data = [[1, np.datetime64('NaT')]]
df = pd.DataFrame(data=data, columns=["no", "dt"])

def encoder(val):
if isinstance(val, (type(pd.NaT), type(np.nan))):
return None
return bytes(val)

def decoder(val): # we do not need that for this task
return val

async def main(df: pd.DataFrame):

conn: asyncpg.Connection=await asyncpg.connect('postgresql://connection_string')

try:
await conn.set_type_codec(
'timestamp',
encoder=encoder,
decoder=decoder,
schema='pg_catalog',
format='binary'
)
res=await conn.copy_records_to_table('test', records=df.values.tolist(), columns=df.columns.values.tolist(), schema_name='sg')
print(res)
except ValueError as e:
print(e.args)
finally:
await conn.close()

asyncio.get_event_loop().run_until_complete(main(df))
```
I am getting following error:
```log
....
File "asyncpg/protocol/protocol.pyx", line 482, in copy_in
File "asyncpg/protocol/protocol.pyx", line 429, in asyncpg.protocol.protocol.BaseProtocol.copy_in
File "asyncpg/protocol/codecs/base.pyx", line 192, in asyncpg.protocol.protocol.Codec.encode
File "asyncpg/protocol/codecs/base.pyx", line 178, in asyncpg.protocol.protocol.Codec.encode_in_python
File "asyncpg/pgproto/./codecs/bytea.pyx", line 19, in asyncpg.pgproto.pgproto.bytea_encode
TypeError: a bytes-like object is required, not 'NoneType'
```

For me it is unclear what kind of bytes should ```encoder``` return in case of Null value, I have tried to investigate it backwards: select a null value from original table with custom decoder, but it seems that ```asyncpg``` just amends that method in case of null values at all. So it is still unclear what kind of data should be passed.

I have already tried to return following values at ```encoder```:
- ```b''```
- ```b'\x01'```
another error occurs:
```log
insufficient data left in message
````

Bắt đầu với các mục stack trace của copy_in và Codec.encode trong asyncpg/protocol/protocol.pyx và asyncpg/protocol/codecs/base.pyx, sau đó kiểm tra asyncpg/pgproto/./codecs/bytea.pyx. Tái hiện ví dụ về set_type_codec và copy_records_to_table với các giá trị null của pandas và NumPy; được xem là hoàn tất khi hành vi null mong đợi cho đường dẫn này đã được xác lập và được bao phủ.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.