Suppress RuboCop offenses

This commit suppresses the following RuboCop offenses:

```console
$ bundle exec rubocop
(snip)

Offenses:

lib/rubocop/cop/rails/not_null_column.rb:95:23: C: [Correctable] Style/MultipleComparison:
Avoid comparing a variable with multiple items in a conditional, use Array#include? instead.
            return if type.value == :virtual || type.value == 'virtual'
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
lib/rubocop/cop/rails/not_null_column.rb:96:24: C: [Correctable] Style/MultipleComparison:
Avoid comparing a variable with multiple items in a conditional, use Array#include? instead.
            return if (type.value == :text || type.value == 'text') && database == MYSQL
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

296 files inspected, 2 offenses detected, 2 offenses autocorrectable
```

lib/rubocop/cop/rails/not_null_column.rb

@@ -46,6 +46,9 @@ class NotNullColumn < Base
         MSG = 'Do not add a NOT NULL column without a default value.'
         RESTRICT_ON_SEND = %i[add_column add_reference].freeze
 
+        VIRTUAL_TYPE_VALUES = [:virtual, 'virtual'].freeze
+        TEXT_TYPE_VALUES = [:text, 'text'].freeze
+
         def_node_matcher :add_not_null_column?, <<~PATTERN
           (send nil? :add_column _ _ $_ (hash $...))
         PATTERN
@@ -92,8 +95,8 @@ def on_block(node)
 
         def check_column(type, pairs)
           if type.respond_to?(:value)
-            return if type.value == :virtual || type.value == 'virtual'
-            return if (type.value == :text || type.value == 'text') && database == MYSQL
+            return if VIRTUAL_TYPE_VALUES.include?(type.value)
+            return if TEXT_TYPE_VALUES.include?(type.value) && database == MYSQL
           end
 
           check_pairs(pairs)