default value on PK field does not work

Describe the bug
default value on PK field does not work

To Reproduce

MODEL:

class User(Model):
	id = fields.BigIntField(pk=True, default=6)

Action:

user = User()
user.save()

Report:

Traceback (most recent call last):
  File "handle_request", line 96, in handle_request
    if TYPE_CHECKING:
  File "/Users/apple/SynologyDrive/code/rena_sanic/account/views.py", line 18, in get
    await user.save()
  File "/Users/apple/opt/anaconda3/envs/sanic/lib/python3.9/site-packages/tortoise/models.py", line 960, in save
    await executor.execute_insert(self)
  File "/Users/apple/opt/anaconda3/envs/sanic/lib/python3.9/site-packages/tortoise/backends/base/executor.py", line 226, in execute_insert
    insert_result = await self.db.execute_insert(self.insert_query, values)
  File "/Users/apple/opt/anaconda3/envs/sanic/lib/python3.9/site-packages/tortoise/backends/base_postgres/client.py", line 34, in _translate_exceptions
    return await self._translate_exceptions(func, *args, **kwargs)
  File "/Users/apple/opt/anaconda3/envs/sanic/lib/python3.9/site-packages/tortoise/backends/asyncpg/client.py", line 86, in _translate_exceptions
    raise IntegrityError(exc)
tortoise.exceptions.IntegrityError: null value in column "id" violates not-null constraint
DETAIL:  Failing row contains (null, 0, 86, 0, null, 0, 0, 2022-10-05 04:39:59.639835+00, 2022-10-05 04:39:59.639953+00).

Expected behavior
A clear and concise description of what you expected to happen.

Additional context
Add any other context about the problem here.

Start by reproducing the model and save sequence shown in the issue against PostgreSQL. Read tortoise/models.py around save and tortoise/backends/base/executor.py around execute_insert, then verify that the configured primary-key default is included in the insert rather than producing a null id; no test file is identified in the report.