Fixes rubocop#6888.

This PR fixes the following error for `Rails/ActiveRecordOverride`
when there is no `parent_class` to match against.

```ruby
% cat app/models/user.rb
class User
  def initialize; end

  def create; end
end
```

```console
% rubocop app/models/user.rb --only Rails/ActiveRecordOverride -d
For /Users/dannyiachini/github/jpw/rubocop_ex: configuration from /Users/dannyiachini/github/jpw/rubocop_ex/.rubocop.yml
Default configuration from /Users/dannyiachini/.gem/ruby/2.5.3/gems/rubocop-0.67.2/config/default.yml
Inspecting 1 file
Scanning /Users/dannyiachini/github/jpw/rubocop_ex/app/models/user.rb
An error occurred while Rails/ActiveRecordOverride cop was inspecting /Users/dannyiachini/github/jpw/rubocop_ex/app/models/user.rb:6:2.
undefined method `const_name' for nil:NilClass
/Users/dannyiachini/.gem/ruby/2.5.3/gems/rubocop-0.67.2/lib/rubocop/cop/rails/active_record_override.rb:42:in `on_def'

(snip)

1 file inspected, no offenses detected

1 error occurred:
An error occurred while Rails/ActiveRecordOverride cop was inspecting /Users/dannyiachini/github/jpw/rubocop_ex/app/models/user.rb:6:2.
Errors are usually caused by RuboCop bugs.
Please, report your problems to RuboCop's issue tracker.
https://github.com/rubocop-hq/rubocop/issues

Mention the following information in the issue report:
0.67.2 (using Parser 2.6.2.0, running on ruby 2.5.3 x86_64-darwin17)
```