Fix an error for Metrics/ClassLength

[koic]

Follow #8122.

This PR fixes the following error for Metrics/ClassLength when using or assignment (||=).

% cat example.rb
Constant ||= Struct.new {}

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

Inspecting 1 file
Scanning /Users/koic/src/github.com/koic/rubocop-issues/8122/example.rb
An error occurred while Metrics/ClassLength cop was inspecting
/Users/koic/src/github.com/koic/rubocop-issues/8122/example.rb:1:0.
undefined method `class_definition?' for nil:NilClass
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/cop/metrics/class_length.rb:43:in `on_casgn'
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/cop/commissioner.rb:99:in `block (2 levels) in trigger_responding_cops'
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/cop/commissioner.rb:152:in `with_cop_error_handling'
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/cop/commissioner.rb:98:in `block in trigger_responding_cops'
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/cop/commissioner.rb:97:in `each'
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/cop/commissioner.rb:97:in `trigger_responding_cops'
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/cop/commissioner.rb:70:in `on_casgn'
/Users/koic/src/github.com/rubocop-hq/rubocop-ast/lib/rubocop/ast/traversal.rb:60:in `block in on_or_asgn'
/Users/koic/src/github.com/rubocop-hq/rubocop-ast/lib/rubocop/ast/traversal.rb:60:in `each'
/Users/koic/src/github.com/rubocop-hq/rubocop-ast/lib/rubocop/ast/traversal.rb:60:in `on_or_asgn'
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/cop/commissioner.rb:72:in `on_or_asgn'
/Users/koic/src/github.com/rubocop-hq/rubocop-ast/lib/rubocop/ast/traversal.rb:14:in `walk'
/Users/koic/src/github.com/rubocop-hq/rubocop/lib/rubocop/cop/commissioner.rb:85:in `investigate'

This PR have also tweaked the highlight area of offense to more eligible location for count the class block length.

Before:

Foo = Class.new do
^^^ Class has too many lines. [6/5]

After:

Foo = Class.new do
      ^^^^^^^^^^^^ Class has too many lines. [6/5]

I haven't added the fixing to the changelog because #8122 hasn't been released yet.

---

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.

[marcandre]

Oh, right!
It's an interesting pattern, I wonder if it is common?
Maybe it should be abstracted (later) in rubocop-ast, so that for any "assignment" we can get the assignment block with one method call, wdyt?

[koic]

Good point. I'm not sure yet, but I think we can investigate a similar case and consider abstracting it to RuboCop AST (later).