Fix an error for Rails/UniqueValidationWithoutIndex

This PR is fix an error for `Rails/UniqueValidationWithoutIndex`.

Author: ydah

changelog/fix_an_error_for_rails_unique_validation_without_index.md

@@ -0,0 +1 @@
+* [#1021](https://github.com/rubocop/rubocop-rails/pull/1021): Fix an error for `Rails/UniqueValidationWithoutIndex`. ([@ydah][])

lib/rubocop/cop/rails/unique_validation_without_index.rb

@@ -31,7 +31,7 @@ class UniqueValidationWithoutIndex < Base
         RESTRICT_ON_SEND = %i[validates].freeze
 
         def on_send(node)
-          return if uniqueness_part(node).falsey_literal?
+          return if uniqueness_part(node)&.falsey_literal?
           return if condition_part?(node)
           return unless schema
 

spec/rubocop/cop/rails/unique_validation_without_index_spec.rb

@@ -2,12 +2,24 @@
 
 RSpec.describe RuboCop::Cop::Rails::UniqueValidationWithoutIndex, :config do
   context 'without db/schema.rb' do
-    it 'does nothing' do
-      expect_no_offenses(<<~RUBY)
-        class User < ApplicationRecord
-          validates :account, uniqueness: true
-        end
-      RUBY
+    context 'when using `uniqueness: true`' do
+      it 'does nothing' do
+        expect_no_offenses(<<~RUBY)
+          class User < ApplicationRecord
+            validates :account, uniqueness: true
+          end
+        RUBY
+      end
+    end
+
+    context 'when using other validation helpers' do
+      it 'does nothing' do
+        expect_no_offenses(<<~RUBY)
+          class User < ApplicationRecord
+            validates :name, presence: true
+          end
+        RUBY
+      end
     end
   end