Allow account email to be null in the database

When using login with Stellar, the email address remains empty. This was
causing issues since the email is also required to be unique, in effect
only a single Stellar account could exist.

Resolved this by allowing the email column to be null, since the unique
requirement does not cover null values. An empty email will now be
stored as null value.

For non-Stellar logins, an email address is still required.