Convert default for email address fields to '' instead of NULL

[jmcameron]

There is currently some inconsistency in default for the email fields. In some cases, it is '' and in others it is NULL. I suggest converting the default to ''. That we we can always treat email fields as strings. Hopefully this will simplify email address checking.

Note that this means the migration code will need to convert all NULL email fields to '' for existing records.