Allow remember token to be stored in the database with a default value of the empty string or 0 or something else that is not null, if a user so chooses.

Checking is_null would make it so the user would not be remembered after the first login if you have the remember_token field default in the database set to anything but null.
Delete existing password reminder tokens in database.
Merge pull request #5302 from rickywiens/4.2

[4.2] Allow remember token to be stored in the database with a non-null default