The migration to 0.7.0 produces errors

The 0.7.0 version changed the database schema quite substantially.

One of these changes was to make the remote_accounts.password column NOT NULL DEFAULT(''). However, when applying the 0.7.0 migration to the table, the migration fails because the column contains NULL values. This is due to the fact that, unlike the string type, the types.CypherText type used in the RemoteAccount model has a default value of NULL, instead of ''.

The migration should be changed take this into account, and first update the column to convert all NULL values to empty strings.

A similar fix will be needed for the crypto_credentials.private_key column, which also uses types.CypherText in its model.