FIX: Migrate old blank override for upload site settings.

If the depercated SiteSetting.logo_url was set to "" previously, it will
not have been migrated by the onceoff job which we added. As such, we
need a migration to migrate the blank override so that the default for
certain upload site settings will not be used.

db/migrate/20190315025804_migrate_blank_override_for_upload_site_settings.rb

@@ -0,0 +1,40 @@
+class MigrateBlankOverrideForUploadSiteSettings < ActiveRecord::Migration[5.2]
+  def up
+    {
+      'logo_url' => 'logo',
+      'logo_small_url' => 'logo_small',
+      'digest_logo_url' => 'digest_logo',
+      'mobile_logo_url' => 'mobile_logo',
+      'large_icon_url' => 'large_icon',
+      'favicon_url' => 'favicon',
+      'apple_touch_icon_url' => 'apple_touch_icon',
+      'default_opengraph_image_url' => 'opengraph_image',
+      'twitter_summary_large_image_url' => 'twitter_summary_large_image',
+      'push_notifications_icon_url' => 'push_notifications_icon'
+    }.each do |old_name, new_name|
+      if DB.query_single("SELECT 1 FROM site_settings WHERE name = '#{old_name}' AND value = ''").present? &&
+         DB.query_single("SELECT 1 FROM site_settings WHERE name = '#{new_name}'").empty?
+
+        ActiveRecord::Base.connection.execute <<~SQL
+        INSERT INTO site_settings (
+          name,
+          data_type,
+          value,
+          created_at,
+          updated_at
+        ) VALUES (
+          '#{new_name}',
+          18,
+          '',
+          CURRENT_TIMESTAMP,
+          CURRENT_TIMESTAMP
+        )
+        SQL
+      end
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end