sql: properly handle NULL hashedPassword column in system.users
Fixes #48769. Before this change, attempts to log in as a user with a NULL value in their "hashedPassword" column in the system.users table would cause the server to crash. This was because `retrieveUserAndPassword` was not properly handling NULL values. This change fixes this. Release note (bug fix): Manually writing a NULL value into the system.users table for the "hashedPassword" column will no longer cause a server crash during user authentication.
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