Fix an error for Style/GuardClause

[koic]

This PR fixes the following error for Style/GuardClause when using heredoc as an argument of raise in then branch and it does not have else branch:

def func
  if condition
    raise <<~MESSAGE
      oops
    MESSAGE
  end
end

% bundle exec rubocop --only Style/GuardClause -d
(snip)

An error occurred while Style/GuardClause cop was inspecting /Users/koic/src/github.com/koic/rubocop-issues/guard_clause/example.rb:1:0.
undefined method `source_range' for nil:NilClass
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/style/guard_clause.rb:209:in `autocorrect_heredoc_argument'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/style/guard_clause.rb:191:in `autocorrect'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/style/guard_clause.rb:179:in `block in register_offense'

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• 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.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[bbatsov]

Thanks!