FIX: Change default for IncomingEmail#created_via to 0 (unknown) and make NOT NULL (#11782)

Follow up to https://review.discourse.org/t/dev-add-created-via-column-to-incomingemail-pr-11751/18366/6

db/migrate/20210121001720_change_incoming_email_created_at_null.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class ChangeIncomingEmailCreatedAtNull < ActiveRecord::Migration[6.0]
+  def up
+    # 0 signifies unknown
+    DB.exec("UPDATE incoming_emails SET created_via = 0 WHERE created_via IS NULL")
+    change_column_default :incoming_emails, :created_via, 0
+    change_column_null :incoming_emails, :created_via, false
+  end
+
+  def down
+    change_column_null :incoming_emails, :created_via, true
+  end
+end