Add change_null for change_table

To change a NOT NULL constraint `reversible`.

When changing a NOT NULL constraint, we use `ActiveRecord::ConnectionAdapters::SchemaStatements#change` method that is not reversible, so `up` and `down` methods were required. Actually, we can use `change_column_null` method if only one constraint changed, but if we want to change multiple constarints with ALTER QUERY, `up` and `down` methods were required.

activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb

@@ -496,6 +496,7 @@ module ActiveRecord
     #     t.timestamps
     #     t.change
     #     t.change_default
+    #     t.change_null
     #     t.rename
     #     t.references
     #     t.belongs_to
@@ -615,6 +616,16 @@ module ActiveRecord
         @base.change_column_default(name, column_name, default_or_changes)
       end
 
+      # Sets or removes a NOT NULL constraint on a column.
+      #
+      #  t.change_null(:qualification, true)
+      #  t.change_null(:qualification, false, 0)
+      #
+      # See {connection.change_column_null}[rdoc-ref:SchemaStatements#change_column_null]
+      def change_null(column_name, null, default = nil)
+        @base.change_column_null(name, column_name, null, default)
+      end
+
       # Removes the column(s) from the table definition.
       #
       #  t.remove(:qualification)

activerecord/test/cases/migration/change_table_test.rb

@@ -281,6 +281,13 @@ module ActiveRecord
         end
       end
 
+      def test_change_null_changes_column
+        with_change_table do |t|
+          @connection.expect :change_column_null, nil, [:delete_me, :bar, true, nil]
+          t.change_null :bar, true
+        end
+      end
+
       def test_remove_drops_single_column
         with_change_table do |t|
           if RUBY_VERSION < "2.7"