fix: set default content type as text for message (#3060)

Fixes #2974

db/migrate/20210923132659_set_content_type_text_for_the_old_messages.rb

@@ -0,0 +1,5 @@
+class SetContentTypeTextForTheOldMessages < ActiveRecord::Migration[6.1]
+  def change
+    change_column_null(:messages, :content_type, false, 0)
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2021_09_22_082754) do
+ActiveRecord::Schema.define(version: 2021_09_23_132659) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_stat_statements"
@@ -519,7 +519,7 @@ ActiveRecord::Schema.define(version: 2021_09_22_082754) do
     t.boolean "private", default: false
     t.integer "status", default: 0
     t.string "source_id"
-    t.integer "content_type", default: 0
+    t.integer "content_type", default: 0, null: false
     t.json "content_attributes", default: {}
     t.string "sender_type"
     t.bigint "sender_id"