Fix an error for Metrics/ClassLength

Follow rubocop#8122.

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

```console
% 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:

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

After:

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

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

lib/rubocop/cop/metrics/class_length.rb

@@ -39,8 +39,13 @@ def on_class(node)
         end
 
         def on_casgn(node)
-          _scope, _name, block_node = *node
-          check_code_length(node) if block_node.class_definition?
+          if node.parent&.assignment?
+            block_node = node.parent.children[1]
+          else
+            _scope, _name, block_node = *node
+          end
+
+          check_code_length(block_node) if block_node.class_definition?
         end
 
         private

spec/rubocop/cop/metrics/class_length_spec.rb

@@ -174,7 +174,7 @@ class Test
     it 'registers an offense' do
       expect_offense(<<~RUBY)
         Foo = Class.new do
-        ^^^ Class has too many lines. [6/5]
+              ^^^^^^^^^^^^ Class has too many lines. [6/5]
           a = 1
           a = 2
           a = 3
@@ -190,7 +190,7 @@ class Test
     it 'registers an offense' do
       expect_offense(<<~RUBY)
         Foo = ::Class.new do
-        ^^^ Class has too many lines. [6/5]
+              ^^^^^^^^^^^^^^ Class has too many lines. [6/5]
           a = 1
           a = 2
           a = 3
@@ -206,7 +206,21 @@ class Test
     it 'registers an offense' do
       expect_offense(<<~RUBY)
         Foo = Struct.new(:foo, :bar) do
-        ^^^ Class has too many lines. [6/5]
+              ^^^^^^^^^^^^^^^^^^^^^^^^^ Class has too many lines. [6/5]
+          a = 1
+          a = 2
+          a = 3
+          a = 4
+          a = 5
+          a = 6
+        end
+      RUBY
+    end
+
+    it 'registers an offense when inspecting or equals (`||=`) for consntant' do
+      expect_offense(<<~RUBY)
+        Foo ||= Struct.new(:foo, :bar) do
+                ^^^^^^^^^^^^^^^^^^^^^^^^^ Class has too many lines. [6/5]
           a = 1
           a = 2
           a = 3