[ci skip] Update all guides with `null: false` change in migrations

- We no longer add `null: false` for timestamps columns as per
  rails/rails@a939506.
- Followup of
  14867b1a9a.

guides/source/active_record_basics.md

@@ -361,7 +361,7 @@ class CreatePublications < ActiveRecord::Migration[5.0]
       t.string :publisher_type
       t.boolean :single_issue
 
-      t.timestamps null: false
+      t.timestamps
     end
     add_index :publications, :publication_type_id
   end

guides/source/active_record_migrations.md

@@ -41,7 +41,7 @@ class CreateProducts < ActiveRecord::Migration[5.0]
       t.string :name
       t.text :description
 
-      t.timestamps null: false
+      t.timestamps
     end
   end
 end
@@ -287,7 +287,7 @@ class CreateProducts < ActiveRecord::Migration[5.0]
       t.string :name
       t.text :description
 
-      t.timestamps null: false
+      t.timestamps
     end
   end
 end
@@ -847,7 +847,7 @@ class CreateProducts < ActiveRecord::Migration[5.0]
       create_table :products do |t|
         t.string :name
         t.text :description
-        t.timestamps null: false
+        t.timestamps
       end
     end
 

guides/source/association_basics.md

@@ -105,13 +105,13 @@ class CreateBooks < ActiveRecord::Migration[5.0]
   def change
     create_table :authors do |t|
       t.string :name
-      t.timestamps null: false
+      t.timestamps
     end
 
     create_table :books do |t|
       t.belongs_to :author, index: true
       t.datetime :published_at
-      t.timestamps null: false
+      t.timestamps
     end
   end
 end
@@ -136,13 +136,13 @@ class CreateSuppliers < ActiveRecord::Migration[5.0]
   def change
     create_table :suppliers do |t|
       t.string :name
-      t.timestamps null: false
+      t.timestamps
     end
 
     create_table :accounts do |t|
       t.belongs_to :supplier, index: true
       t.string :account_number
-      t.timestamps null: false
+      t.timestamps
     end
   end
 end
@@ -180,13 +180,13 @@ class CreateAuthors < ActiveRecord::Migration[5.0]
   def change
     create_table :authors do |t|
       t.string :name
-      t.timestamps null: false
+      t.timestamps
     end
 
     create_table :books do |t|
       t.belongs_to :author, index: true
       t.datetime :published_at
-      t.timestamps null: false
+      t.timestamps
     end
   end
 end
@@ -222,19 +222,19 @@ class CreateAppointments < ActiveRecord::Migration[5.0]
   def change
     create_table :physicians do |t|
       t.string :name
-      t.timestamps null: false
+      t.timestamps
     end
 
     create_table :patients do |t|
       t.string :name
-      t.timestamps null: false
+      t.timestamps
     end
 
     create_table :appointments do |t|
       t.belongs_to :physician, index: true
       t.belongs_to :patient, index: true
       t.datetime :appointment_date
-      t.timestamps null: false
+      t.timestamps
     end
   end
 end
@@ -308,19 +308,19 @@ class CreateAccountHistories < ActiveRecord::Migration[5.0]
   def change
     create_table :suppliers do |t|
       t.string :name
-      t.timestamps null: false
+      t.timestamps
     end
 
     create_table :accounts do |t|
       t.belongs_to :supplier, index: true
       t.string :account_number
-      t.timestamps null: false
+      t.timestamps
     end
 
     create_table :account_histories do |t|
       t.belongs_to :account, index: true
       t.integer :credit_rating
-      t.timestamps null: false
+      t.timestamps
     end
   end
 end
@@ -349,12 +349,12 @@ class CreateAssembliesAndParts < ActiveRecord::Migration[5.0]
   def change
     create_table :assemblies do |t|
       t.string :name
-      t.timestamps null: false
+      t.timestamps
     end
 
     create_table :parts do |t|
       t.string :part_number
-      t.timestamps null: false
+      t.timestamps
     end
 
     create_table :assemblies_parts, id: false do |t|
@@ -388,13 +388,13 @@ class CreateSuppliers < ActiveRecord::Migration[5.0]
   def change
     create_table :suppliers do |t|
       t.string  :name
-      t.timestamps null: false
+      t.timestamps
     end
 
     create_table :accounts do |t|
       t.integer :supplier_id
       t.string  :account_number
-      t.timestamps null: false
+      t.timestamps
     end
 
     add_index :accounts, :supplier_id
@@ -472,7 +472,7 @@ class CreatePictures < ActiveRecord::Migration[5.0]
       t.string  :name
       t.integer :imageable_id
       t.string  :imageable_type
-      t.timestamps null: false
+      t.timestamps
     end
 
     add_index :pictures, [:imageable_type, :imageable_id]
@@ -488,7 +488,7 @@ class CreatePictures < ActiveRecord::Migration[5.0]
     create_table :pictures do |t|
       t.string :name
       t.references :imageable, polymorphic: true, index: true
-      t.timestamps null: false
+      t.timestamps
     end
   end
 end
@@ -518,7 +518,7 @@ class CreateEmployees < ActiveRecord::Migration[5.0]
   def change
     create_table :employees do |t|
       t.references :manager, index: true
-      t.timestamps null: false
+      t.timestamps
     end
   end
 end