[Fix #834] Fix an error for Rails/WhereNotWithMultipleConditions

Fixes #834.

This PR fixes an error for `Rails/WhereNotWithMultipleConditions`
when using `where.not` with empty hash literal.

changelog/fix_an_error_for_rails_where_not_with_multiple_conditions.md

@@ -0,0 +1 @@
+* [#834](https://github.com/rubocop/rubocop-rails/issues/834): Fix an error for `Rails/WhereNotWithMultipleConditions` when using `where.not` with empty hash literal. ([@koic][])

lib/rubocop/cop/rails/where_not_with_multiple_conditions.rb

@@ -45,7 +45,7 @@ def on_send(node)
 
         def multiple_arguments_hash?(hash)
           return true if hash.pairs.size >= 2
-          return false unless hash.values[0].hash_type?
+          return false unless hash.values[0]&.hash_type?
 
           multiple_arguments_hash?(hash.values[0])
         end

spec/rubocop/cop/rails/where_not_with_multiple_conditions_spec.rb

@@ -44,4 +44,10 @@
       User.where.not(trashed: true).where.not(role: 'admin')
     RUBY
   end
+
+  it 'does not register an offense for `where.not` with empty hash literal' do
+    expect_no_offenses(<<~RUBY)
+      User.where.not(data: {})
+    RUBY
+  end
 end