UniqueValidationWithoutIndex raises NoMethodError on empty validates

[jamiemccarthy]

The Rails/UniqueValidationWithoutIndex cop assumes any validates node has at least one argument. It starts its work by scanning the arguments to see if there is a uniqueness part to the directive. But if there are no arguments, it crashes.

(To be clear, validates requires arguments and its presence without arguments is invalid.)

Expected behavior

On a class with a validates directive that has no arguments, no rubocop-rails cop should crash.

(Bonus points if some rubocop-rails cop flags it as incorrect. Currently none does.)

Actual behavior

Rails/UniqueValidationWithoutIndex crashes with a NoMethodError:

# rubocop app/models/foo.rb
Inspecting 1 file
An error occurred while Rails/UniqueValidationWithoutIndex cop was inspecting /workspaces/cq/app/models/foo.rb:13:2.
To see the complete backtrace run rubocop -d.
.

1 file inspected, no offenses detected

1 error occurred:
An error occurred while Rails/UniqueValidationWithoutIndex cop was inspecting /workspaces/cq/app/models/foo.rb:13:2.
Errors are usually caused by RuboCop bugs.
Please, report your problems to RuboCop's issue tracker.
https://github.com/rubocop/rubocop/issues

Mention the following information in the issue report:
1.56.3 (using Parser 3.2.2.3, rubocop-ast 1.29.0, running on ruby 3.2.2) [aarch64-linux]

# rubocop app/models/foo.rb -d
...
Scanning /workspaces/cq/app/models/foo.rb
An error occurred while Rails/UniqueValidationWithoutIndex cop was inspecting /workspaces/cq/app/models/foo.rb:13:2.
undefined method `hash_type?' for nil:NilClass
/usr/local/bundle/gems/rubocop-rails-2.21.1/lib/rubocop/cop/rails/unique_validation_without_index.rb:128:in `uniqueness_part'
/usr/local/bundle/gems/rubocop-rails-2.21.1/lib/rubocop/cop/rails/unique_validation_without_index.rb:35:in `on_send'
/usr/local/bundle/gems/rubocop-1.56.3/lib/rubocop/cop/commissioner.rb:143:in `public_send'
/usr/local/bundle/gems/rubocop-1.56.3/lib/rubocop/cop/commissioner.rb:143:in `block (2 levels) in trigger_restricted_cops'
...

Steps to reproduce the problem

Add the single line

  validates

to any ApplicationRecord class.

RuboCop version

# rubocop -V
1.56.3 (using Parser 3.2.2.3, rubocop-ast 1.29.0, running on ruby 3.2.2) [aarch64-linux]
  - rubocop-capybara 2.19.0
  - rubocop-minitest 0.31.1
  - rubocop-performance 1.19.1
  - rubocop-rails 2.21.1