Fix errors of Rails/UniqueValidationWithoutIndex

[Tietew]

Rails/UniqueValidationWithoutIndex dies with following error:

When the table does not exist:

An error occurred while Rails/UniqueValidationWithoutIndex cop was inspecting <filename>.
undefined method `each_node' for nil:NilClass
.../lib/rubocop/cop/mixin/active_record_helper.rb:14:in `find_set_table_name'

When defining module instead of class:

An error occurred while Rails/UniqueValidationWithoutIndex cop was inspecting <filename>.
undefined method `with_column?' for nil:NilClass
.../lib/rubocop/cop/mixin/active_record_helper.rb:55:in `resolve_relation_into_column'

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[koic]

Thanks!