Fix duplicate key errors in PostDeployMigrateUserExternalMailData migration

`email_provider` by default is NULL, and if a user had not logged the
value would remain NULL. Upgrading to GitLab 10.0 would lead to a
PG::UniqueViolation because the post-deploy migration would attempt
to reinsert the entry because the NULL comparison between
`users.email_provider` and `user_synced_attributes_metadata.email_provider`
would never match.

Closes #38246

changelogs/unreleased/sh-fix-issue-38246.yml

+---
+title: Fix duplicate key errors in PostDeployMigrateUserExternalMailData migration
+merge_request:
+author:
+type: fixed

db/migrate/20170828135939_migrate_user_external_mail_data.rb

@@ -33,7 +33,7 @@ class MigrateUserExternalMailData < ActiveRecord::Migration
           SELECT true
           FROM user_synced_attributes_metadata
           WHERE user_id = users.id
-          AND provider = users.email_provider
+          AND provider = users.email_provider OR (provider IS NULL AND users.email_provider IS NULL)
         )
         AND id BETWEEN #{start_id} AND #{end_id}
       EOF

db/post_migrate/20170828170502_post_deploy_migrate_user_external_mail_data.rb

@@ -33,7 +33,7 @@ class PostDeployMigrateUserExternalMailData < ActiveRecord::Migration
           SELECT true
           FROM user_synced_attributes_metadata
           WHERE user_id = users.id
-          AND provider = users.email_provider
+          AND provider = users.email_provider OR (provider IS NULL AND users.email_provider IS NULL)
         )
         AND id BETWEEN #{start_id} AND #{end_id}
       EOF