Fix Performance/RedundantStringChars cop error in case of implicit receiver

[viralpraxis]

echo 'chars.size' | be rubocop --only Performance/RedundantStringChars --stdin bug.rb -d

Inspecting 1 file
Scanning bug.rb
An error occurred while Performance/RedundantStringChars cop was inspecting bug.rb:1:0.
undefined method `begin_pos' for nil
lib/rubocop/cop/performance/redundant_string_chars.rb:76:in `correction_range'
lib/rubocop/cop/performance/redundant_string_chars.rb:62:in `block in on_send'

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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.

[koic]

Thanks!