Remove not_null constraint and default for public_email

to make it uniform with other secondary emails
Change API to return nil for public_email

Changelog: changed

db/post_migrate/20211018123316_remove_not_null_constraint_and_default_for_public_email.rb

+# frozen_string_literal: true
+
+class RemoveNotNullConstraintAndDefaultForPublicEmail < Gitlab::Database::Migration[1.0]
+  enable_lock_retries!
+
+  def up
+    change_column_null :users, :public_email, true
+    change_column_default :users, :public_email, from: '', to: nil
+  end
+
+  def down
+    # There may now be nulls in the table, so we cannot re-add the constraint here.
+    change_column_default :users, :public_email, from: nil, to: ''
+  end
+end

db/schema_migrations/20211018123316

+fcee17f25ad3a8b6fb56c23b72978f4a6952200264d285b9e3df4b5e7733e8f6
\ No newline at end of file

db/structure.sql

@@ -20153,7 +20153,7 @@ CREATE TABLE users (
     encrypted_otp_secret_salt character varying,
     otp_required_for_login boolean DEFAULT false NOT NULL,
     otp_backup_codes text,
-    public_email character varying DEFAULT ''::character varying NOT NULL,
+    public_email character varying,
     dashboard integer DEFAULT 0,
     project_view integer DEFAULT 0,
     consumed_timestep integer,

ee/spec/requests/api/members_spec.rb

@@ -498,7 +498,7 @@ RSpec.describe API::Members do
         end
 
         context 'when members have no public_email' do
-          it { is_expected.to include(a_hash_including('email' => '')) }
+          it { is_expected.to include(a_hash_including('email' => nil)) }
         end
       end
     end