Fix an error when running --only or --except options

[koic]

This PR fixes an error when running rubocop --only or rubocop --except options without cop name argument.

Before

% rubocop --only
undefined method `empty?' for nil:NilClass
/Users/koic/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/rubocop-0.82.0/lib/rubocop/options.rb:87:in `block in add_cop_selection_csv_option'
/Users/koic/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/rubocop-0.82.0/lib/rubocop/options.rb:206:in `block in option'
/Users/koic/.rbenv/versions/2.7.1/lib/ruby/2.7.0/optparse.rb:1589:in `block in parse_in_order'
/Users/koic/.rbenv/versions/2.7.1/lib/ruby/2.7.0/optparse.rb:1575:in `catch'

After

% rubocop --only
--only argument should be [COP1,COP2,...].

The same is true for rubocop --except.

---

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.
• 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.

[bbatsov]

Nice catch!