Fix a crash in Lint/EmptyConditionalBody

```ruby
var =
  unless object.action value:, other:
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Avoid `unless` branches without a body.
    condition || other_condition # This is the value of `other:`, like so:
                                 # `other: condition || other_condition`
  end
```

In the condition above, the variable assignment is tricking the
`CommentsHelp#find_end_line` method into calling an end method of
`Parser::Source::Map::Variable`. Other `Parser::Source::Map` subclasses
do not respond to respond to the `#end` method as well, won't hurt to
check.

changelog/fix_fix_a_crash_in_lint_empty_conditional_body.md

@@ -0,0 +1 @@
+* [#11712](https://github.com/rubocop/rubocop/pull/11712): Fix a crash in `Lint/EmptyConditionalBody`. ([@gsamokovarov][])

lib/rubocop/cop/mixin/comments_help.rb

@@ -71,7 +71,7 @@ def find_end_line(node)
         elsif (next_sibling = node.right_sibling) && next_sibling.is_a?(AST::Node)
           next_sibling.loc.line
         elsif (parent = node.parent)
-          parent.loc.end ? parent.loc.end.line : parent.loc.line
+          parent.loc.respond_to?(:end) && parent.loc.end ? parent.loc.end.line : parent.loc.line
         else
           node.loc.end.line
         end

spec/rubocop/cop/lint/empty_conditional_body_spec.rb

@@ -317,6 +317,19 @@ class Foo
     RUBY
   end
 
+  context '>= Ruby 3.1', :ruby31 do
+    it 'registers an offense for multi-line value omission in `unless`' do
+      expect_offense(<<~RUBY)
+        var =
+          unless object.action value:, other:
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Avoid `unless` branches without a body.
+            condition || other_condition # This is the value of `other:`, like so:
+                                         # `other: condition || other_condition`
+          end
+      RUBY
+    end
+  end
+
   context 'when AllowComments is false' do
     let(:cop_config) { { 'AllowComments' => false } }