[Fix rubocop#43] REMOVE change_column_null from BulkChangeTable

The `change_column_null` method does not have corresponding method to
use in the suggested `change_table` version.

We should remove it from the offended methods list.

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Bug fixes
+
+* [#43](https://github.com/rubocop-hq/rubocop-rails/issues/43): REMOVE `change_column_null` method from `BulkChangeTable` cop offenses. ([@anthony-robin][])
+
 ## 2.0.1 (2019-06-08)
 
 ### Changes
@@ -18,3 +22,4 @@
 [@koic]: https://github.com/koic
 [@andyw8]: https://github.com/andyw8
 [@buehmann]: https://github.com/buehmann
+[@anthony-robin]: https://github.com/anthony-robin

lib/rubocop/cop/rails/bulk_change_table.rb

@@ -129,7 +129,6 @@ class BulkChangeTable < Cop
 
         POSTGRESQL_COMBINABLE_ALTER_METHODS = %i[
           change_column_default
-          change_column_null
         ].freeze
 
         def on_def(node)

spec/rubocop/cop/rails/bulk_change_table_spec.rb

@@ -114,9 +114,18 @@ def change
 
     it 'registers an offense when including combinable alter methods' do
       expect_offense(<<~RUBY)
+        def change
+          change_column_default :users, :name, false
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ You can use `change_table :users, bulk: true` to combine alter queries.
+          change_column_default :users, :address, false
+        end
+      RUBY
+    end
+
+    it 'does not register an offense for `change_column_null`' do
+      expect_no_offenses(<<-RUBY.strip_indent)
         def change
           change_column_null :users, :name, false
-          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ You can use `change_table :users, bulk: true` to combine alter queries.
           change_column_null :users, :address, false
         end
       RUBY
@@ -139,6 +148,13 @@ def change
     it 'does not register an offense' \
        'when including combinable alter methods' do
       expect_no_offenses(<<~RUBY)
+        def change
+          change_column_default :users, :name, false
+          change_column_default :users, :address, false
+        end
+      RUBY
+
+      expect_no_offenses(<<-RUBY.strip_indent)
         def change
           change_column_null :users, :name, false
           change_column_null :users, :address, false