Fix an incorrect auto-correct for Style/MultilineWhenThen

[koic]

This PR fixes the following incorrect auto-correct for Style/MultilineWhenThen when line break for multiple condidate values of when statement.

% cat example.rb
case condition
when :foo,
     :bar then nil
end

% rubocop -a
(snip)

E

Offenses:

example.rb:3:11: E: Lint/Syntax: unexpected token kNIL
(Using Ruby 2.7 parser; configure using TargetRubyVersion parameter,
under AllCops)
     :bar nil
          ^^^
example.rb:3:11: C: [Corrected] Style/MultilineWhenThen: Do not use then
for multiline when statement.
     :bar then nil
          ^^^^

1 file inspected, 2 offenses detected, 1 offense corrected

% cat example.rb
case condition
when :foo,
     :bar nil
end

% ruby -c example.rb
example.rb:3: syntax error, unexpected `nil', expecting `then' or ',' or
';' or '\n'
     :bar nil

---

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.