Fix an error for Style/SingleLineDoEndBlock

[koic]

This PR fixes the following error for Style/SingleLineDoEndBlock when using single line do...end with no body

$ echo 'foo do end' | bunlde exec rubocop --stdin test.rb -d --only Style/SingleLineDoEndBlock
(snip)

An error occurred while Style/SingleLineDoEndBlock cop was inspecting
/Users/koic/src/github.com/rubocop/rubocop/test.rb:1:0.
Expected a Parser::Source::Range, Comment or RuboCop::AST::Node, got NilClass
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/corrector.rb:111:in `to_range'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/corrector.rb:120:in `check_range_validity'
/Users/koic/.rbenv/versions/3.3.0-dev/lib/ruby/gems/3.3.0+0/gems/parser-3.2.2.4/
lib/parser/source/tree_rewriter.rb:398:in `combine'
/Users/koic/.rbenv/versions/3.3.0-dev/lib/ruby/gems/3.3.0+0/gems/parser-3.2.2.4/
lib/parser/source/tree_rewriter.rb:207:in `wrap'
/Users/koic/.rbenv/versions/3.3.0-dev/lib/ruby/gems/3.3.0+0/gems/parser-3.2.2.4/
lib/parser/source/tree_rewriter.rb:243:in `insert_after'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/style/single_line_do_end_block.rb:45:in `block in on_block'

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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.