Fix an error for Style/SelectByRegexp

[koic]

This PR fixes the following error for Style/SelectByRegexp when block body is empty:

array.select { }

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

An error occurred while Style/SelectByRegexp cop was inspecting /Users/koic/src/github.com/koic/rubocop-issues/empty_body/example.rb:1:0.
undefined method `begin_type?' for nil:NilClass
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/style/select_by_regexp.rb:89:in `on_send'

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• 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).
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• 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.