[Fix #7759] Fix an error for Layout/LineLength

[koic]

Fixes #7759.

This PR fixes the following error for Layout/LineLength cop when using lambda syntax that argument is not enclosed in parentheses.

% cat example.rb
-> x {}

% bundle exec rubocop --only Layout/LineLength -d
(snip)

An error occurred while Layout/LineLength cop was inspecting
/Users/koic/src/github.com/koic/rubocop-issues/7759/example.rb:1:0.
undefined method `begin_pos' for nil:NilClass
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/cop/layout/line_length.rb:127:in
`check_for_breakable_block'
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/cop/layout/line_length.rb:71:in
`on_block'
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/cop/commissioner.rb:57:in
`block (2 levels) in trigger_responding_cops'
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/cop/commissioner.rb:136:in
`with_cop_error_handling'

---

Before submitting the PR make sure the following are checked:

• 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.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

Thanks!