[Fix #9606] Fix an error for Layout/IndentationConsistency

[koic]

Fixes #9606.

This PR fixes the following error for Layout/IndentationConsistency when using access modifier at the top level.

% cat example.rb
public

def foo
end

% bundle exec rubocop --only Layout/IndentationConsistency example.rb -d
(snip)

Scanning /Users/koic/src/github.com/koic/rubocop-issues/9606/example.rb
An error occurred while Layout/IndentationConsistency cop was inspecting
/Users/koic/src/github.com/koic/rubocop-issues/9606/example.rb:1:0.
undefined method `source_range' for nil:NilClass
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/layout/indentation_consistency.rb:165:in
`base_column_for_normal_style'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/layout/indentation_consistency.rb:181:in
`check_normal_style'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/layout/indentation_consistency.rb:174:in
`check'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/layout/indentation_consistency.rb:129:in
`on_begin'

---

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!