Update users on public email index

After removing not_null constraint for public_email !72514 (merged), we should update the index to save storage space.

Currently the index is set up like this:

CREATE INDEX index_users_on_public_email ON users USING btree (public_email) WHERE ((public_email)::text <> ''::text);

User records with empty string are not indexed, but records with NULL value will be indexed..

The vast majority of users have no public email set, so this index will start growing in size.

We need to add OR public_email IS NOT NULL to the partial index condition.

I think we need to add AND public_email IS NOT NULL.