Remove not_null constraint and default for public_email

What does this MR do and why?

Solves #341055 (closed)

This MR defines public_email the same way as the other 2 secondary emails: public_email character varying

Reasons:

We can observe an error occurring since code to set public_email to an empty-string was removed.

User secondary emails are defined in SQL as follows:

• commit_email character varying
• notification_email character varying
• public_email character varying DEFAULT ''::character varying NOT NULL

For Database review:

== 20211018123316 RemoveNotNullConstraintAndDefaultForPublicEmail: migrating ==
-- change_column_null(:users, :public_email, true)
   -> 0.0026s
-- change_column_default(:users, :public_email, {:from=>"", :to=>nil})
   -> 0.0115s
== 20211018123316 RemoveNotNullConstraintAndDefaultForPublicEmail: migrated (0.0142s)

== 20211018123316 RemoveNotNullConstraintAndDefaultForPublicEmail: reverting ==
-- change_column_null(:users, :public_email, false, "")
   -> 0.0323s
-- change_column_default(:users, :public_email, {:from=>nil, :to=>""})
   -> 0.0155s
== 20211018123316 RemoveNotNullConstraintAndDefaultForPublicEmail: reverted (0.0479s)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.