Handle missing password authentication cleanly

[aryansk]

Summary

Avoid an internal AttributeError when PostgreSQL requests cleartext password authentication but the connection parameters do not contain a password.

Changes

• detect a missing password before building the cleartext password message
• raise an asyncpg InterfaceError with an actionable message instead of calling .encode() on None
• add regression coverage for connecting to a password-authenticated role without supplying a password

Test plan

python -m unittest tests.test_connect.TestAuthentication.test_auth_password_cleartext_without_password

Fixes #1286