Does two things:
1. Revert "Revert "Add unique constraint to email branding domain""
This reverts commit af9cb30ef3.
2. Don’t allow empty string in email branding domain
Columns with multiple `null`s can have a uniqueness constraint. Columns
with multiple empty string values are not considered unique.
This commit:
- removes any duplicate empty string values
- casts empty strings to null string any time these columns are updated
---
Squashed into this single commits because these two things are not
atomic as individual commits.
Brandings with a domain set should be considered canonical. It doesn’t
make sense to have the same domain set on multiple different email
brands – you can’t tell which one to use.
