Connect with passwordless DSN does not work

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Basically as the title says: Trying to connect with a DSN that contains no password does not work

DSN in question:

`
@property
def dsn(self) -> str:
return f"postgresql+asyncpg://{self.pguser}@{self.pghost}:{self.pgport}/{self.pgdatabase}"
`

results, when used with

`
def get_db_url() -> str:
config = get_config()
return config.postgres.dsn

engine = create_async_engine(
get_db_url(),
echo=True,
future=True,
)
`

in the error:

`
ERROR: Traceback (most recent call last): File "asyncpg/protocol/coreproto.pyx", line 168, in asyncpg.protocol.protocol.CoreProtocol._process__auth File "asyncpg/protocol/coreproto.pyx", line 577, in asyncpg.protocol.protocol.CoreProtocol._parse_msg_authentication File "asyncpg/protocol/coreproto.pyx", line 672, in asyncpg.protocol.protocol.CoreProtocol._auth_password_message_cleartext AttributeError: 'NoneType' object has no attribute 'encode'
`

Reproduce the failure with the passwordless PostgreSQL DSN shown and the create_async_engine entry point. Trace the password-authentication path around the reported _auth_password_message_cleartext traceback; done means a passwordless connection no longer raises the NoneType encode error, with coverage for this case.

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