Change condition of users on public email index

What does this MR do and why?

This MR solves for #344806 (closed).

In !72514 (merged) we removed not_null constraint for public_email.

The vast majority of users have no public email set. This MR is changing the condition of the index_users_on_public_email to save storage space by not indexing User records with public_email set to empty string or NULL.

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);

With this MR it will be:

CREATE INDEX index_users_on_public_email ON users USING btree (public_email) WHERE (((public_email)::text <> ''::text) AND (public_email IS NOT NULL));

MR acceptance checklist

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